Depends in part how much I'm getting paid and what the target audience is. Short answer "Current FF and Safari, IE back through 6, and any other remotely modern browser will pretty much be fine without any additional effort."
Long answer, since testing on wacky browsers is one of those things I do for fun (got the original Mosaic running once, though of course without HTTP1.1 support you can't do anything with it):
For the most part, the percentage of people running NS4 or other similarly archaic browsers is zero at this point, and even IE5 is down to a fraction of a percent, so I generally code so that it looks right in FF, Safari, and IE7, and (depending on the job and the pay) is at least nearly the same in IE6.
IE5.x, I'll glance to make sure it's usable, but unless there's a specific requirement for it I generally won't go past that--many major sites are varying levels of broken in IE5 now anyway, so clearly the "big guys" don't care, either. Besides, if it's ok in IE6, 5.5 is generally fine with some visual glitches without any additional work and 5.0 is at least usable.
I consider NS4 easy; since it won't see any stylesheet with a target set, it simply won't see any of my stylesheets. And since I always code with semantically correct (and preferably also simple and tidy) (X)HTML, NS4 users get a very dry, but entirely functional experience. Sure, it's not pretty, but all the info is there.
I suppose were somebody to specifically ask for it I'd code a simple stylesheet for NS4 that tiptoed around its extreme limitations, but so far as I know nobody cares anymore.
IE5 Mac is usually in the same boat as IE5 Win. Technically it's superior, but it has enough glitches that if it looks fine I'm just lucky, but I'm also not going to fix for it so long as the site is usable. There are a handful of IE5 holdouts, but unless it was a Mac-centric site catering to a technically backward audience, I can't imagine there being enough of them to be worth more than a cursory glance.
As for all the other modern ones, if it's valid and looks ok in current FF and Safari, it's probably going to look good in older versions of same, Opera, Konqueror, other Webkit browsers, etc. I'll check in Opera and maybe an old version of FF, and fix if there's anything obvious, but thus far I've never done anything fancy enough to significantly affect either.
I haven't yet had a job that paid for any specific attention to iPhone or other mobile browsers, but it'd be interesting to work with. I figure if it's usable at 640x480 the iPhone browser can probably handle it ok, and other mobile browsers will hopefully just ignore the stylesheets so get the dry but usable version.
Not that anybody uses them, but the Wii browser is easy, since it's just Opera with a funky interface; the PS3 browser is some other proprietary engine, but it seems to handle standards well enough that it requires no extra efforts on the sites I've checked with it thus far. I haven't tried the DS browser yet, but I assume it's about the same as the Wii.
...now, if it's a pet project, I'll test in EVERYTHING. Might not fix, but at least to see if it's usable. My record is testing in NS and IE back through 3 (no stylesheets for both, so that's easy; IE4 is ugly but I made sure it was usable at least, and wasn't nuts enough to bother trying to hide the stylesheets from it), Opera back to 6 (generally fine), iCab (current is just Webkit so that's easy; older builds have basically no CSS support so just ugly but usable), Lynx (easy with the good markup), IE5 Mac (after a few fixes it looked surprisingly nice), and I also visually confirmed "same as all modern browsers" in screenshots of Konqueror and several other random Linux browsers. Oh, and WebTV (there was an emulator for OS9 you could use to test), which is in pretty much the same boat as IE4.
The one that stymied me was the in game browser from EVE Online, which uses a crude proprietary engine. It's of course not intended for anything but in-game websites, but it is possible to browse the greater 'net with it, and I got a player to send me some screenshots. Unfortunately I couldn't figure out any easy way to hide stylesheets from it, and since floats are utterly broken in it, it ends up looking worse than with no styles. Not like it matters, I was just going nuts for fun.
