Update: a plumber in Dayton showed me the trick of marking pipe cuts with a sharpie before soldering, cut my rework from 5 callbacks to zero | Contractor Chat - Free To Speak
Does the marker actually help with fit up or is it more about knowing where you already cut? I've seen guys do that and swear by it, but I still check every joint twice before I fire up the torch. Whatever keeps the callbacks at zero is worth doing though, so if it works for you then keep at it.