I think there's a sort of zen balance to be sought. IF your bike is set up right (in the world of ideals, etc) you should be able to find a comfortable way to settle with your pelvis in a neutral position, sit-bones on the saddle, proper knee-over-pedal-spindle and a relaxed sort of position to the arms/shoulders so you're in the happy medium between stretched out and curled up, where your arms/shoulders/chest are kept loose to allow for proper breathing etc. If you're tensing too much to compensate for things (tightening the core too much to maintain a pelvis-neutral position, or shifting hands often because you're putting too much weight on them) you may need to adjust your bike.