I would say not to. I always assume that user's of Opera, Konqueror, SeaMonkey, etc... are pretty savvy and will upgrade as soon as they can. Also, just as a general thought, I think we will eventually see a level of conformance to the W3C specs so that we can stop hacking and coding for every separate browser version (I'm looking at you IE6).
I think it's important to code cross-browser. If only 1% of your users use opera, and you have 100 visitors per day, that's one visitor every day that your inconveniencing by providing for example div positioning that doesn't work in opera the same it would in firefox. That's 365 times a year that someone gets disappointed when they view your site before even looking at the content. 1% doesn't seem like much, but over a large period of time it adds up.