By default, of course, Visual Studio creates separate bin folders for Debug and Release builds. We are having some minor issues dealing with those from the perspective of ex
Occasionally one may run into a particularly-nasty uninitialized memory problem that only occurs with a release build. If you are unable to maintain (as ChrisF suggests) separate names for your debug vs. release binaries it's really easy to loose track of which binary you're currently using.
Additionally, you may find yourself tweaking the compiler settings (i.e. optimization level, release-with-debug symbols for easy profiling, etc.) and it's much easier to keep these in order with separate folders.
It's all a matter of personal preference though - which is why Visual Studio makes it easy to change the options.