Think... It's not so hard to consciously consider your grip for a moment WHILE you are gripping at his Gi. I think these kind folks are right about the tightness of your fists. Relax a bit even if you need to remind yourself with a que. Unless you
very recently started the game or the gripping part of it???
I often tell folks that rock climbing is (in my experience) the best thing for hand strength and toughness. You think it's tough to keep a grip on cloth? Try solid rock!
The results are similar, new guys will hang on with some kind of "death" grip the entire time without even realizing how rigid they actually are and wonder why their parts are aching later...