When I flew from Belize City to Flores last October, there were taxi drivers waiting at the airport to ferry people around. I was going to Tikal and they prices they were quoting were all in US dollars. Not sure what it's like outside the airport, though. I only spent a few days in Guatemala but it seemed like most of the prices in Flores were in Quetzales. Around Tikal, though, they seemed to take US $ ok (i.e. Tikal Canopy Tours quoted prices in US). Probably best to have a bit of both.