Ok, let's take an example. You have 2 stacks of TFB. You use an Overpower, it procs TFB, and then you get another proc. Now you're at 4 stacks, and you have the opportunity to Overpower again, but you're going to rage cap on the next Melee attack, and you have CS available.
Obviously, the onus now is to dump the Rage on the Heroic Strike, inside the CS window, not to press OP again so you have a chance to get another stack. It's as simple as that.
This can be applied to every damn situation. You only need to worry about how much damage you do inside a CS window, it's that simple.