For a long time I have tried to hold onto users who are reluctant or unable to upgrade their browser, at the cost of not supporting modern browsers to the extent that they should be supported. This has been an unfortunate case of certain industries struggling to catch up and still relying on older technology. As CSS3 makes many interface elements easier to create, you need to decide if you prefer to advance technology or grasp at the past. Depending on the audience you are developing for, you may not be able to write-off older browsers as quickly as you would like. But take a look at where you place the emphasis. Are you pushing technology and providing progressive enhancement or graceful degradation, or are you limiting yourself by your lowest denominator?
Periodically review browser trends on your sites. I see an extremely low percentage of IE6 and 7 on the sites I manage. I wish I could have said this several years ago. There are still some on IE8 but most on modern releases.
Most vanilla HTML/CSS should render relatively fine on IE6/7/8, it's when you start adding the enhancements that things fall apart. The technologies you use in development will obviously be driven by the type of site and will influence which browsers can be supported. I am currently able to find peace with CSS3, by using CSSPIE to keep things straight in IE, and everything else can use plain styling. At some point you need to draw a line. If an old browser can't handle it, I am not going to spend much time trying to figure out a workaround, at the end of the day it's not worth the effort, not when new features can be implemented with much less work.