Broadsides for cruisers and sci ships, frontal for escorts. You can mix it up a little too. One DBB for a cruiser or sci ship gives you a little added dps on frontals. For sci ships, most of your powers need to be within the front arc to use anyway. But sci ships don't often run enough weapon power to fuel more than one DBB.
All of what has been said is true, naturally. If you're in a ship with a crappy turn rate, as I've just learned, you're better off with beam arrays. I was using a dual beam bank on a Galaxy-X... it didn't go well.