As usual, depends....
Since only a few hard hitting pros actually hit 90 mph forehands regularly, those that do would be trying to hit especially hard ...harder than normal groundie speed, so they tend to grip tighter.
But remember, the tighter you grip, the more you whole kinetic swing get's off it's axxis, and some tend to arm thru on those shots.
A generally flowing forehand can easily go 85mph, when everything is working, so maybe it's not too important to go for that extra, as that extra 5-15 just might go out more often, hurt your arms, or even ruin your confidence.
How hard? Hard enough not to allow the racket to spin, but not hard enough to ruin your full swing.