A HighRanking Forum thread named 8/10 - Need Advice, keyword research - getting 10/10 shows how quickly SEOs like, keyword research expert Dan Thies, agrees with well known SEO woman, Jill Whalen that when optimizing pages, one should not worry about keyword density figures.
I'll quote Dan's reply here:
As Jill says, keyword density means nothing. You're going to spend a bunch of time chasing your tail with tools like that.
If you want to play around and test/tweak stuff, test your TITLE tags and the first heading (H1) on the page. You can make changes, and see how your rankings change. A few rounds of testing on each should give you 90% of what you're going to get by fiddling with the HTML code on a given web page.
For the rest of the copy, just work on using keywords naturally, and make it work as copy. Do that well, and you're at about 95% if not higher. Now forget about the other 5% and use your internal links to help searchers find the right page for the right search terms.
Ron Carnell offers good advice as well; "The key to any scientific testing is repeatability."