I train my forearms on back/bi day at the end of the workout. I have also been doing some on shoulder day. I wanted to see if 2 times a week would help. So far, twice a week seems to be helping. (To short a time period to know for sure.)
Whatever day you do them, I suggest you do them at the end of the workout. (Especially if it's on back/bi day.) That way, your grip won't hinder the rest of your workout.
.02,
Joker