Do you miss out small tsu?
Forget when to lengthen a vowel?
Use the wrong voicing?
Help may be at hand from Purdue University. It is an old project (the last update was 2002) but they have available custom dictionaries for Japanese IME systems. I downloaded the extra dictionaries for Kotoeri. They also have files for Windows XP.
After managing to extract them (the self-extracting archive would no longer run on my system) I needed to repair some filenames that didn’t survive the extracting process. The Japanese characters had turned to mojibake. I then dragged them to the Dictionaries folder in my home folder’s library. Lastly all I had to do was turn them on in Kotoeri’s Prefrences. The English translation of the dialogue boxes has made doing this so much easier in Leopard.
Now when I type an extra character or omit one, Kotoeri will also suggest words near to the one I typed. You can recognise these words because they have their correct reading in brackets beside them. This furigana will also be inserted. By doing it this way I think you are more likely to remember the correction and not become over-reliant on the spellcheck.
Japanese obviously don’t have these problems. Usually if you can say it properly you can spell it properly (in kana). Japanese is a wonderful language in this respect. This is a helpful idea for learners of the language.
As yet I am in two minds. Do I just use the normal dictionary and retype when I can’t get the correct kanji because I misspelled? Or do I open the extra dictionaries when this probably overpopulates the menus and I have to edit the furigana out?