If you're lining your subs up with your mains right on top of them, delay can help you line them up so the impulse response matches and gives you better impact and less negative phase interaction. But you probably need better then millisecond resolution for it to be useful. If you're trying to align subs that are not directly near your mains, alignment could be a moot point. Say you have a sub or a cluster of subs center stage and your mains are off to the left and right. Or maybe flown. You can only get the subs to align with the tops at one point in the room. So if you line them up so they're all in time at mix position, they're still going to be out everywhere else in room. In my particular room, I have a row a double 18 subs across the front of the stage with my tops/mains flown about 15 feet above the ground. I don't bother with time alignment between subs and tops because it will only be correct in one spot anyway. When I ground stack my tops on my subs, I'll tweak the alignment to increases output at the crossover frequency by reducing phase cancelation with the proper delay.
