You can buy them predrilled if you want too, usually they charge .25/hole.
I drill them as I use them, don't want any more holes in them than necessary (these posts are STRONG, but any hole will weaken it a bit). Never know where you might need to use them, or what the wire spacing might be. Perimeter is 3 wire, internal 1 wire. I drilled holes in an old steel U post for a story board. Welded a short piece of flat over the top end so I can hang it on the posts after driving them ALL in, the length of the run. Then just go down the line with a cordless drill and a couple of fully charged batteries in your pocket, with a 3/16" carbide tipped cement bit. I've drilled several thousand holes with the same bit, still working great. I use a hammer drill, seems to speed it up a little. Before using a carbide drill though, I could burn up bits in no time, AND wear out the man trying to make the durn thing work.
Typically I drill all the posts, then come back with my wire ties and hang the wires... speeds up the process. I don't use cotter pins.... just cut a whole soup can full (easy to carry with you down the line) of 7-8" long pieces of 14 ga. soft galvanized, stick through the post underneath the wire, bend over the top of the wire loosely and twist tie it 2 rounds behind the post. Don't tie tightly... that HT wire is supposed to be able to stretch the whole length of the fence.
Install end posts... pull up 1 wire to establish the line... drive in all line posts... drill all posts... pull in other wires and "pretension slightly"... hang all wires in a single pass... final tension wires to preference
I try to standardize my inventory. All end posts are 4 1/2" x 8' long fiberglass, including the cross bar for H brace. Use only 1 1/4" x 6' for line posts, even on 1 wire electric (anything much lighter and it won't support the weight and tension of a polywire reel for daily breaks anyway). I've been installing only 3 wire and 1 wire fences (don't have sheep or goats), so when I drill for 1 wire fence, I'm putting in only a single hole, where the middle wire would be on a 3 wire fence. That way, even if I take posts out, they will still work no matter where I need to use one.