My experience is "it depends" when the perfect depth is concerned, seldom found any deeper then 12' deep, that doesnt mean they are not down there, but its hard to fish that deep close to structure without getting hung up.
From there I have caught them in all different depths, from a ft deep during the spawn to the deeper depths in the hottest part of the summer.
Almost always close to cover but sometimes during cooler fall weather you can catch quite a few trolling in open water, at different depths and speeds and of course different colors of jigs.
I like to place cover between the 14' depth to some almost showing on the surface, especially when the water is a little low.
Good Luck!
