Is pre-drilling really necessary?

Yes, with any plastics PVC or polycarbonate, the material will expand and contract with temperature changes.  The predrilling of fastener holes 3/16" larger than the screw shank diameter will allow for that expansion and contraction while eliminating the risk of spider cracks in polycarbonate or old canning in PVC. ONA also offers an 1 1/4" one step that has flanges on the shank that will cut the hole to the required diameter thus eliminating the pre-drilling step.