the CO2 does not have to necessarily be let out, but more like, so no pressure builds up. If you are storing in mason jars, pressurizing them can cause the to crack / break. I don't think you'd ever get enough to pop a plastic bag, I have yet to see it from off gassing. The CO2 actually I think would slow down the aging process a bit because the oxygen is not on the beans oxidizing them. Same reason we pack them in vac bags etc, to keep the oxygen off them.
If you are shipping, leave a pinhole in the plastic bag, because a bag filling up like a pillow, first time the postal monkeys drop the box or slap something heavy on top of it, that bag is going to pop and you have beans busted all over in the box now, if they rip the box, now you lose your product.
Aaron