If you are on the search process then different diving depths is the way you should go, you are searching for them so you need to have your lures in a stair like pattern, once you have located the depth at which the fish are holding then you can switch baits and have them all running at the same depth. The numeber of strikes and what you see in your finder tells you at which depth the fish are holding for that particular area.
Many don 't see trolling as a real fishing technique, I do not agree with such opinion, after all you still have to find the fish, find the right speed that fish are liking, selecting the right lures and so on; it also gives you the oportunity to locate places that attract the fish to them. I found what now is one of my best spots in my home lake by trolling.