That's probably the best way to do this; write the posts in your favorite word processor or text editor, use the spelling/grammar check tools that come with that, then cut/paste into the forum. An extra step, but it's worth it. I go with that extra step even in forums that have all those tools, and even when writing email.
All that cut & paste is too much work, IMO. Admittedly, some grammar & spelling on boards can be atrocious, but that's just part of it, so why not just go with the spontaneity of using the online editor box? I personally find spell checkers to be inadequate, because they don't (and can't, at least for now) catch situations such as "PRINCIPAL / PRINCIPLE" or "LOOSE / LOSE". The spelling will be correct according to the checker, but the word is still wrong. Neither spelling checkers nor grammar checkers will catch those kinds of definition-based spelling errors. I must, however concede that spell checkers do help eliminate most errors that result from sloppy typing. (I.E.: adn, teh, fo, et. al.)