I'm still very new at it, but my experience has been similar to the original post markp99. However, each cache has it's own subleties and adds bits of experience. I think it's the experience that tells us when to stay on the trail or take a B-line to the cache.
The distance to the cache is also a critical factor. The farther from the cache the more I'm inclined to stay on the trail path. Almost all caches are off the trails so when I start to parallel the cache within 100 feet and the trail seems to continue away, I'll look for any signs of traffic or a clear path to the cache.
Thinking about it some more, it seems to also depend on the time of year, at least in heavily forested areas. The tactics would seem to change depending on the season. I've only had experience in the nastiest summer heat and insect abundance. Does anyone have any thoughts about approaches in winter, spring, or fall?