There's a couple of reasons to consider glassbedding a GI fiberglass stock. First, although most newer stocks fit most receivers well, some do not. I have seen GI fiberglass stocks with fore and aft slop at the receiver legs, and also with poor triggerguard lockup tension. Another reason to consider glassing is not for bedding, but for stiffening of the forend. If you compare torsional rigidity of the forends of even a standard weight GI wood stock versus a GI glass stock (hold/stabliize the stock, no receiver and barrel, with one hand against your body and torque the forend with the other), you will probably think that the glass stock is a little whippy up front compared to the wood. It's easy enough to fix, takes quite a bit of bedding material and attention paid to gas cylinder and op rod guide clearances. Some people use glass cloth in addition to acraglas, also some use aluminum channel or steel rods.