Normally you should hold your just enough so you don't loose it when playing. More than that will add unwanted stress on your muscles.

BUT! Having learn to play guitar with metal .. I can honestly say that I was getting a better crunchy metal sound when I added more resistance on the pick.

I will hold my pick tighter when playing "aggressive" songs and release some tension when playing in a strumming kinda rhythm.

So you have to find the middle ground where you can get a tone you like and have less tension in your hand.

You might want to let less of the pick standing out when you play. That way with less pick, you'll have less resistance from the string. That might help you having to hit the string with less force to get the same sound.