I upload the same title/keywords/description to all agencies, perhaps i'm missing out on some subtle improvisation, but they all seem to accept pretty much the same set of keywords if you follow some guidelines:
1) you'll find yourself deleting keywords at istock during their disambiguation
2) the agencies that don't want plurals seem strip them if you have the singular
3) Do not start the title or description with 'a', 'an' or '1' etc.
4) Make your description at least 7 words long.
5) Add at least 10 keywords, but not more than 50.
6) Duplicate all the keywords used in the title and description into the keywords field (some agencies don't search on the title field)
7) Take care with special characters, letters with accents, quotes and even apostrophes (i try to leave them out all together) these can either truncate the description or lead to unusual characters being displayed - so always have a quick check that all your metadata has been read in correctly after you have uploaded.
have a look at
http://blog.keywordsmart.com for some info about each agency and its handling of keywords
and for streamlining microstock uploads have a look at
http://microstockinsider.com/guides/uploading-microstock-images and the rest of the workflow section
I'd stick with 10-12 agencies - any more than that and the low earning sites don't justify the income (your photos might be better than mine, your time might be worth less etc)