Why is it inappropriate to dip rice into soy sauce when eating sushi?
It is not how a Sushi Chef is intended. Each step in making Nigiri is carefully thought out and designed to achieve a perfect taste in your mouth. Therefore if you dip the rice into the soy sauce, you may not achieve the taste the Sushi Chef intended.
In other words, it is not how the Chef would like you to eat Nigiri.
For the sake of this discussion, we'll look at the making and the design of Maguro, Tuna Nigiri.
Here are the steps I would take to make Maguro Nigiri.
1. Slicing a piece Maguro
Mainly, there are three different kinds of Maguro for Sushi; Bluefin, Big Eye and Yellowfin Tuna. They all taste different, and each part of the fish tastes different. Belly has more fat content than the back and the tail. This means I must cut each part in different lengths and thicknesses. For example, I would cut O-Toro (fattiest belly) very thin, thinner than back meat because a thick belly piece would taste overwhelming from the fat. It requires an experienced eye and taste bud of a Sushi Chef to determine how to cut a piece of Maguro for nigiri.
2. Applying wasabi
I think wasabi goes well with Maguro. In fact, wasabi tastes better with Maguro than any other fish. (it's my personal opinion, as well as most of the Sushi Chefs I've worked with in the past seventeen years in California. Just as a reference, Jiro Ono said the same thing, too :)
So, I would apply more wasabi to a Maguro Nigiri than Hamachi Nigiri. In other words, an experienced Sushi Chef would apply a different amount of wasabi to a Nigiri, based on the type of fish and part of the fish. I would apply more wasabi to Toro than regular Maguro. Wasabi's aroma is wonderful creating a great harmony with the taste of Maguro.
3. Forming Rice
Based on the size and the thickness of the Maguro I sliced, I would estimate how much Sushi Rice I would need to form a nigiri, as I imagine how the slice of Maguro would taste. After applying wasabi on the piece of Maguro, I would form rice applying pressure. Ideally, a perfectly formed rice should have some tiny pockets of air inside. At the same time, each grain of the rice should stay intact. The reason for the air pockets is so that when you put Nigiri in your mouth, the rice will spread apart inside, releasing the flavor of rice vinegar, sugar, salt and sweetness and Umami.
4. Applying soy sauce on the piece of Maguro
The final step is to apply a thin coat of Soy Sauce on the top piece of Maguro, using a brush. Wasabi, Maguro and Soy Sauce create a perfect balance of taste and aroma. Soy Sauce erases the nice aroma of Wasabi, so just a small appropriate amount on the fish side is good enough. (note that not all sushi chefs season the nigiri with Soy Sauce before they serve them.)
These are the main steps in making Maguro Nigiri.
Each step aims to achieve a perfect balance of taste and aroma. Everything is carefully calculated and designed.
Thus, this is what will happen when you dip the rice part of your Nigiri into the soy sauce.
1. Rice will fall apart before you put it in your mouth
When you dip the rice into soy sauce, it will quickly absorb soy sauce and starts to fall apart, before you can put it into your mouth. As mentioned above, Sushi Chef intended the rice to spread apart in your mouth, not on the soy plate, therefore, what you will taste, will be different from what the chef wanted you to taste. Even if you managed to put Nigiri into your mouth without losing a grain of rice, rice will have too much soy sauce, which, will change the flavor balance of the Nigiri.
2. Kill the aroma of Wasabi
Too much soy sauce in the Shari, Sushi Rice will kill the nice aroma of wasabi applied under the piece of Maguro. Remember the Chef's intention of applying more wasabi to Maguro? That will be in vain.
3. Rice will taste different
Sushi Rice is seasoned with Rice Vinegar, Salt and Sugar (some chefs do not use sugar). Soaking the rice side of Nigiri will alter the taste of Shari, which, again, is not what the Chef intended.
So, when you dip the rice into the soy sauce, you are saying;
A) I don't know or care about how the chef intended my Nigiri to taste.
B) I am aware of how the chef intended and designed my Nigiri to taste. Knowing this, I would like to eat it in my way.
Is it inappropriate or wrong to eat that way? I feel the answer depends on the chef. I would not say it is wrong. As I stated above, it's not how Nigiri is designed.
That being said, in conclusion, I would like to say this: Eat your sushi the way you like to eat it. (Kaz Matsune's answer to What's the correct way to eat sushi?)
Through years of my experience, I came to understand that people are free to eat their Sushi in a way that please them. I came to understand that is the best way to eat anything. I came to understand that each person has a different taste bud. I came to understand though some people say Natto (fermented soybeans) smell so foul, yet they love the smell of Blue Cheese. I came to understand that just because I love the taste of Uni, that doesn't mean my clients would love them, too. I came to understand that my opinion as a Sushi Chef matter, but it is not final. I came to understand that I am not always right about Sushi.
If a Sushi Chef gets offended for his customer for not eating his Nigiri the way he intended, that is his ego, not respecting his customer, or thinking he is better than his customer.
I came to understand both the chef and his customers are equal. If I fail to respect my customers, my customers will not respect me either.
A customer should not obey a chef automatically and a chef should not obey his customer automatically either. Sushi Chefs are no better than you, non-chefs, the customers. We know more about Sushi, but that does not make us better. Our job as a Chef, I think, is to guide you and serve you, not force you to change your mind or the way you eat Sushi.
The way to eat Nigiri is just a proposal, an idea, a suggestion and it's up to you to take it or leave it.