The best tool I've ever used for DIY Rolex sizing is a knockoff copy of the Panerai screwdriver. It's 1.6mm, hollow-ground (a.k.a. cut out) tip. They're pretty ubiquitous, but here's
one for sale on eBay for reference.
I prefer to size my own watches because I've had jewelers scratch them before, like others have mentioned. Also, you can really take your time tinkering with the fitment, move/center the clasp, etc. If the loctite is super tight, hit it with a cigarette lighter for a second or two.