In my experience a lot of people ask for either swing, stuff (Old blue eyes), or soft acoustic stuff (Jack Johnson) for the early drinks reception part. It's just background music at that stage, no one is actually listening. Personally, I'd start with that, then have 60s with the buffet (gotta get some Kinks in there), "Cool" music for the milling around / drinking - stuff you like to listen to, but not really gonna dance to. Finally, forget cool, and unleash the cheese from the 80s. Be sure to save a couple of "anthems" like "Don't stop me now" for late on, when people will be singing along with their arms around the stranger next to them like they're long lost brethren.