IIRC, the only browser that is specifically restricted by an OS is IE. Any other browser has little reason to not be current. Note how rare people mention coding for older versions of non-IE browsers.
I wouldn't code for IE6 as I mentioned elsewhere it might not even be charted by the end of this year and will be four versions behind as soon as Win8 hits. If you are coding for IE6 by the end of the year, you are doing so out of habit.
I'm hardly motivated to code for IE7 since it will assume the IE6 position shortly, it's tied to Vista who people vastly avoided and there will likely be two newer and better OSs out shortly since then(Win7 is easily better), and MS is running updates for every version of IE to as current of a version as the OS can go.
Due to Vista's horrendous system requirements, it won't even have life in laptop/netbooks. XP was taking over there when Vista hit and now Win7 is the ideal OS for it. Win8 is even more built for smaller computers. Vista was also released in 2007. Considering the typical lifespan of computers, most people would getting a new computer by now, which means Vista is on it's last legs of whatever legs it had.
To me the question isn't do you code for older browsers, it's do you code for mobile devices? Mobile devices are starting to outsell computers.