Coming from someone who played CS 1.3 , 1.6 and CS:S for years using only a basic 3 button mouse. It really does not matter since the DPI is really only for easy access. I tend to set my mice to 800 DPI when playing games and a lot of Pros have it even lower.
Edit: I don't use the extra buttons on my gaming mice, but if you are willing to setup the macros than it can be a little quicker setting up your load-out.