yes , when I bed a rifle I do both pillars and glass , the pillars keep the stock from "crushing" and the glass makes for a skin tight fit ,if your doing one you may as well do them both.
Also very few companies use fiberglass to bed rifles but some do , bedding is typicaly done with products like (my personal favorit) Devcon plastic steel putty , Marine Tex , JB Weld and other products that are desgined for bedding like Acra-Glass and Steel Bed.