I've done it both ways. If the form is perfect and your riser is cut perfect you don't need it. The bow will show every defect no matter how small if you put the glass straight on a hard surface. The rubber helps keep some imperfections from showing up on the finished bow. It also helps with glue lines by closing any gaps without causing a dry joint in places. This is key for me on the back of the riser and fades area. It's really hard for me to get the riser cut absolutley perfect to the form and the rubber under there takes the worry away. I use c clamps on the riser to the fades and tubes on the limbs. I don't use a riser template because I rarely build more than a couple bows off any form. So I usually trace my riser out using the form as the template. I usually lay the back of the bow on the form. Also, if I use tapers between the riser and the back glass there tends to be a gap at the fades (because I traced the form without tapers) and a rubber cushion helps deal with that too. I'm not a precise perfectionist as some of the guys are. They have more patience, skill and better tools than I do.