I'm about to install this acrylic/fiberglass shower base. It's going on top of a wooden subfloor, with 2x4 walls behind it, then tile on the walls. The instructions that came with the base say to lay down a bed of mortar and set the base on top of it, with the flange right against the studs; they don't say anything about screwing the flange to the studs, but I've seen it done. Would the screws be overkill?
I wouldn't put holes there unless you carefully waterproof them and are 100% sure they won't show. Some panels don't sit down tight on the bottom. You'll also want to be sure not to crack the flange if it's not tight. Use shims if needed.
The mud base, floor caulk and plumbing combined will keep it from moving.
If you have specific questions about the installation of a specific product, and the installation instructions do not address your question, the infinite wisdom of these fine random people on the internet is not the place to start.
Call or email the manufacturer and ask them for guidance.
In general, products without factory pilot holes will use a clip for anchoring, or they will use a shower surround that captures the lip and provides the anchoring.
Drilling holes in that lip will invite water into the wall cavity.