"What I like about controlling the cooler fan with a manual switch is that I can anticipate rising temps and get ahead of the curve a little. If the gauge is already warmish, and there's a long grade up ahead, or I'm turning onto the freeway, I'll flip the switch before things start to heat up." - @Sacto Mitch
This is also my set-up and habit as @Stan Galat alluded. Just something about manually flipping the oil cooler fan switch makes me feel as Master of My Domain (Seinfeld).