Any good engine builder will tell you that you ALWAYS have to retorque the head bolts. Actually, it's a good practice to do it at least twice.
The first time is after the engine is brought up to operating temperature the first time and allowed to cool back down. You will most likely notice quite a bit of slack.
The second time should be after a few hours of normal operation (5 or so). Lots of folks do it when they do the first oil and filter change after the rebuild. If didn't rebuild, but just had the head off for a valve job or something, I'd do it after 3 to 5 hours just to be sure.
This procedure is necessary because the head gasket compresses a little, and the temperature cycling expands and contracts the metal in the block, head and headbolts at slightly different rates.
For the few minutes it takes, it's cheap insurance, and a lot less hassle than a blown head gasket.