Think that might have to be a personal choice there eightfifteen, just going off all my sites (with the exception of a couple of hits from mobile phones)...
Visitor CSS compatability on 5 of my sites:
So between 5 sites, I have only not catered to 2.03% of visitors.
I was surprised at your high rate. #3 of my sites targets a lot of senior citizens (genealogy), and lower end users (teens & volunteer groups/rural)...so if I was to see any huge variance in regards to archaic browser support, I would expect to see it there, but I still dont.
But it still pretty much case specific. Though, I think even in your case, I would still use CSS, just format the page in such a way as to not turn away the % that arent compatable. I've never known too many rural people that get put out by not rendering pretty pages, "just the facts m'am" has usually been their motto. As long as they can read the information they want, the rest is left up to them.