I try to keep speeds to be able to panic brake within the distance I can see around the curves. It's very easy to take corners at a speed that would be a disaster if the road ahead was blocked or full of debris. One of the local riders mentioned the automobile signs for curves -- A sign for a "25 mph" corner is about the speed I can go and see far enough ahead. I could probably go 35+ around that same corner if the pavement was good, but it's too fast to stop.
I'll go full speed on long, straight downhills without driveways and with trimmed back shoulders. That's up to the low 40mph range.
I've practiced very hard braking with just the front brake, so I can stop pretty well. (I once fishtailed when I grabbed both brakes hard. The rear wheel locked up. Letting off both brakes kept me from crashing.)