After you finish a writing, if you click ANYWHERE on the word, it will change the grading FOR THE WORD.
However, each _character_ will have its own grading as you marked.
If you have any of the word's characters in your study list, then these gradings will apply.

For example, if you always mark 大 in 大小 as Wrong, but mark the whole word correct, then 大小 will have a very long time between reviews, but you will start to have writing reviews for 大小 come up.

Hope that's clear!

I think you mean that you will start to have writing reviews for 大 come up.

In fact, if you don't alter the grading, on the iOS app, 大 won't even be submitted as a wrong review if Skritter thinks you know 大 and was only wondering if you know 大小. It figures, hey, he must not have known 大小 had this da4 character in it, but he almost certainly knows 大 itself, so we'll just try 大小 again soon. If you mark 大小 correct, though, Skritter will think you mean you really didn't know 大 and give you that one again.

On the iOS app, usually you should just let the characters that you miss stay wrong, since that tends to produce the best behavior. Skritter can guess what you do and don't know almost all of the time, and the remaining edge cases are better handled automatically-and-wrongly than manually-and-slowly.

I use the the "word-level" grading if I can't come up with the Chinese word to match the definition... if I reveal the pronounciation, I'll usually know how to write each character, but I forgot what the word was...
Writing is the only place where you are not given the Chinese character, so in many ways it is a reverse-definition quiz as well as writing.

Before I just marked one of the characters correct... Skritter would probably figure it out, but this way is more elegant... especially if I put so-so for one of the characters..

pretty much!

Nick's point is that you might score one of the characters wrong just to show that you don't know the word... even though you actually know the character.
The iOS version will see (from your history) that you already know the specific character very well, so the character as wrong, but will submit the word wrong.
You can (subtly) see this by the red of the character up top to be slightly lighter than the normal red.

But sticking to your old way of scoring will work just fine!

That's how I use it!
Also for New words, even when I know the word (such as when I'm back-filling old books & vocabulary), I'll just mark the word as "so-so" since I never mark new words as "OK" on 1st time.