Anywhere I can spin for a fairly long period without interruption from traffic, or traffic controls, and where I don't have to dodge anything... sometimes that means a back street, sometimes it is the MUP, sometimes it is a main arterial road at an odd time of day.

Residential streets don't do it as there are too many "intersections" where a car can suddenly pull out.

Industrial neighborhoods tend to be the best, especially when I am commuting just a bit earlier in the morning than the car crowd.