Best hole saw tip I ever got from an old timer: take the hole saw off the mandrel, drill the pilot hole, put the hole saw back on and saw the hole.
Stops snapped bits, busted teeth (saws and yours) and broken wrists.
Second best hole saw tip from an old timer: Take the pilot bit out of the mandrel, cut a #2 phillips screwdriver shaft (they are usually 1/4 inch make sure first), set screw that into the mandrel. Drill the hole with with the pilot bit in the drill, switch out for the hole saw with screwdriver bit.
Stops broken or bent pilot bits. Tempered steel and its smooth.
Third best hole saw tip from an old timer: When drilling into a live panel or even a dead panel with wires, wrap electric tape about a half inch from the saw teeth. It needs to be thick.
Stops from punching all the way thru the panel and maybe cutting something important.
I should mention one little detail, I hate hole saws. They make ragged sharp holes, they bind and twist, they throw shavings all over the place and they are next to impossible to center. When I started in the trade I worked for a rather cheap contractor who would not buy KO sets, hole saws only. Got laid off and got a job with a contractor that had varibits and KO sets, what a difference. That became my preferred method, but hole saws have a place and I have a kit on the truck.
As for the OP, I do commerical work and prefer panels with blank ends so I can cut my own KOs. It takes no more time to cut the KOs to line up with the pipe then it does to bend the pipes to fit the factory KOs which are NEVER cut at standard unistrut sizes off the wall for some reason. I have cut holes in prepunched panels to line up, just make sure to invest in some chinese money first, aka reducing washers.
One last tip, this is from me: if you are cutting a lot of holes in plaster or tile or cement backer board, get a carbine grit hole saw. Goes through easy and makes a clean hole. They have no teeth but rather look like they are dipped in carbine grit.