I buy 8" wide rolls and cut them to fit whatever quantity I'm looking to seal. All of my cascade bags are 2 oz. and the bags are 9-10" long. I always leave them slightly long because there will be times you will seal the end and then find out there is a ripple in it and it is slowly sucking air back in. Now you have to cut it off and reseal. If you start off with the bag as small as possible and then have a ripple, you will either have to bag less or get a new bag. Also, when trying to shove too much into a bag, you cannot get the end of the bag to be perfectly flat to get the perfect seal.
Believe me, there will be many times you think you have a bag set perfectly and proceed to clamp it down and hit the seal button only to find you had a ripple and need to do it again. Sometimes if the ripple is slight doesn't go to the end, you can re-adjust the bag on the sealer by moving it towards the opening and quickly do a second seal before air gets in.
IMO, Do not be conservative in bagging. Give yourself plenty of length.