I usually adjust the valves about 3 times a year, most of the time they are just fine anyways, but it never hurts to check. I'd adjust them when the engine is broken in after the rebuild.
Depends on what you mean by rebuild, if you messed with the valves id run them prolly an hour or 2 and then check the clearences. If you went to a bigger cam then a 1/2 hour and adjust them.