Drilling Countersink for Acrylic Tub-Shower Surround Fasteners
Countersink the heads if you want, use shims where the tub is away from the framing. use weather resistant screws if you want, the screws will never see water. When I was living in Maryland, we always shimmed the wall to let the drywall go over the shower flange. So with that you really don't need to countersink depending on other conditions, like if a door is in the same wall and you do not want to order custom jambs or add something on site to the jamb.

Since I have been on the west coast for a while I have never seen the wall shimmed, unless I am the one who did it. On the other jobs the drywall was ran to the flange, fast set mud filled the leftover and then it was taped and finished. Looked great to me.

And there has been other jobs where the drywall was run over the flange when the flange was set with roofing nails. Made me cringe, and when the wall was finished you could fairly easily pick up on how the wall curves out from the last stud or at the inside corner how the drywall covered the flange. That does not happen in either the shimming or the packing with fast set.