I voted for Buy DRM books and strip the DRM later, but it is not the complete answer.
As mentioned by others, I first always try to buy non DRM"d versions if availible, and try to support sites that do so.
When I do Buy DRM protected books, I do usually strip the DRM as while I buy Mobi books I also have a Sony Reader, and as such find it easy to convert it and a DRM free archive copy at the same time as while eBook may be cheaper, I do want to be able to access my content in the future on any device I might own.
But the bigger question that I face with eBooks is more often then not whether to get with DRM or not, it is whether or not it is available at all.
Unfortunately only a small fraction of books are available as eBooks and as such, even when you are willing to buy them they are not available. In contrast if you are willing Darktent versions are easily available
Publishers Wake up!!