Not even necessarily good practice. It’s more a matter of personal preference. Many developers don’t care at all about 800x600 because they say too few people actually have this resolution anymore. You decide.
There are good reasons why you can write different stylesheets for different devices. Screen, print, handheld… all no problem with different stylesheets.
Yeah, but let's face it, a lot of people don't actually bother.
*references this site*
It depends on the target audience. With a forum, I wouldn't expect that they would target it for mobile users as well since frankly, would you want to write out a long post on your phone?
Even where you do have separate handheld, screen, print etc stylesheets your page layout will be easier to maintain if at least some of the elements are the same in each. For example if your page will display reasonably at 750px width on the screen then you don't have to redesign it for print.