Use both brakes. Just as you've already discovered, it really depends on conditions. Try to brake harder before turning and less during turns. While turning it is better to use too much rear brake than front - a rear skid will actually help turn the bike while a front skid will likely put you on the ground. (Try not to skid anyway, it erodes the trails). If traction conditions are highly variable, hit the brakes harder in the grippier sections to scrub speed before you hit the slippery sections.
Sounds like you have a pretty good feel for it already - just ignore the idiots telling you not to use the rear brake. You might want to try experimenting with how much brake pressure it takes to skid the tires while going straight so that you can get a better feel for it. Just be careful with the front...