I'm in the DIY camp
Sure, there are good, concientious mechanics out there. And a lot of boneheads that I wouldn't want near my truck.
Also, I like to know exactly how it fit up, that I took my time and did it right, etc. The increased liability of towing comes back to me, the truck owner/driver.
I got a Lennox 4" hole saw at Lowes. Nice. Chucked it in a Milwaukee Super Hole Hawg drill. Went through that bed like butter in about 1. 5 seconds.
And if your going to do a spray in liner, get that done after the install. My Lin-X very nicely filled the gap (with exposed metal edge) between the bed sheetmetal and the B&W.
B&W's instructions are good. I think I measured about 5 times to make sure I had it right before drilling.