One other consideration is the wood you're holding. End grain usually makes a stronger tenon than face grain. But regardless, if the wood isn't very hard then you can have trouble holding either. I do two things that seem to work for me. First I apply a liberal dose of thin CA glue to the tenon to help fortify it and then I tighten the jaws (I'm talking the standard #2 talon jaws with the teeth) until they bite into the wood. But once they start leaving visible bite marks if you go beyond that sometimes you're just crushing the wood which might actually weaken the hold.