I'm not 100% sure about the 340, but I'm fairly certain the rule in my 350 manual says 3lbs/turn, and do not back out past 3 turns from dead nuts tight. Obviously that is meant for use, not necessarily just pressing, but I use that as my rule of thumb for pressing too.
I always loosen my limbs when I press my bows, the idea being taking a little stress off of the limbs and riser since when I press it, it might be under a weird torque compared to drawing by hand, and in general, when I press my bow, I expect to have it under higher tension than when under draw and for a considerably longer time.
I think it's particularly important to relieve a little pressure if you're not using the appropriate press for the destroyer. I'm a firm believer that ONLY limb tip compression presses should be used on destroyers, or most parallel limb bows for that matter. If you're using an old style press with your destroyer, take it as easy as you can.