The only exception would be for keywords that are very difficult to spell. Using a thesaurus might help you come up with some terms you might not have thought of. Try Roget Thesaurus . Have a look at what keywords are actually being used. Sites like Search Voyeur at Magellan and MetaCrawler's Metaspy can tell you what words are being searched for at that exact moment, and more importantly, how searches are done.
Searchers are still for the large part, using only one or two terms for each search.
Dog Pile - Top 200 Search Words from the popular Metasearch engine, offered by EyeScream Interactive. This list is from October 1996, but I fear, for the most part, it's still a reasonably accurate reflection on what people are searching for. They also offer a customized report based on your keywords. Top 100 Search terms each month. After you have your keyword list, send your list to all of your buddies, business associates etc., and listen to their opinion.