I think this is an area in which you should strive to use the Robustness principle as far as is practical (which is good advice for any area of coding). Just because something works today doesn't mean it will work tomorrow: if you're relying on a particular HTML/CSS hack or even if you've just been a little lax in emitting strictly valid code, the next iteration of browsers could well break. Doing it once the right way minimises this problem (though does not entirely mitigate it).
There is a certain element of pragmatism to take here, though. I'd certainly do all I could for a client's site to be valid, but I would be willing to take more risks in my own space.
