If you vent your job,
Whether it has 30/60/90 that job wont have more force on it you still have the pressure acting on the bag an the bag pushing the material 
Normally you would vent at 15psi as that's essentially the same as the pull that's being exerted by the vac on the bag,no point letting the pressure going higher then venting
Most jobs you dont have to vent but if you want to stop core marking its essential
Either that or do 2 pre cured skins and then bond them