Nearly all my baitcast reels are the same line, so the IPT factors are more directly tied to ratio than if you have a wide variety of spools. That being said, I think you have to align the equipment with how you mostly use it. If I could only pick one, I'd lean toward 5.4 (I think Lew's makes some in that range), but that means you're going to be a little slow when burning baits. I use 4.4 for deep cranks, 5.1 for spinnerbaits and smaller/shallow cranks, and 6.2 for much of the rest. If I need to burn a spinnerbait faster, I'll usually select a bait with lighter weight and narrow blades so it doesn't wear me out.
-G