Use straps for all heavy sets of rows/pulldowns/pull ups/rack pulls/shrugs and even deadlifts if your hands aren't big enough to get a solid grip on the bar, even with chalk
Don't use them for loads that don't cause your grip to fail within the target rep range your aiming for
Use straps when you're focusing on higher volume/fatigue loading pulls with a peak contraction at the end of the movement....straps allow you to "forget about" your hands and arms and pull primarily with your back and allow a much better mind muscle connection to the back musculature...which in my opinion, is the often missing link in a lot of trainees back development.
In my opinion, maximize back size and strength will NEVER be achieved with out the use of straps for the majority of your heaviest sets or those requiring higher reps with moderate weight.
Train your forearms separately, if forearm size is of importance to you
The less you have to think about your grip, the more you can concentrate on your back