My experience says, as loosely as possible while still feeling the weight of the disc at the hit and without getting slips. Loose is fluid, flexible, smooth.
Experimenting, I recently found a way to grip very tightly and get MUCH more acceleration through the hit. Probably one of the biggest distance-mechanics breakthroughs that I've had. However, moving through the hit with tight forearm muscles and tendons was NOT a good idea. My brain found a way to exceed my body's limits. Result was pain, ice, and elbow brace. Still on the mend.