It will depend on your personal preference and what you'd like to have in your tank.
My tank is predominately LPS and Softies. I've had 36" over my 90 gallon, with 48" Reefbrite LED's on each side in the past. There would be shadowing on the ends of the aquarium with this setup. However, some like it that way for placement of lower light corals on the ends. I personally like the spread of T5 from end to end, so currently I have a T5/LED hybrid with 48" ATI T5 and 2 Kessil AP700's.
If you search the threads, you'll see lots of pics of various setups.