Honestly, I don't think it ultimately matters much in the grand scheme of things, especially if you're doing a full rebuild with fresh bore, new pistons/rings, valve job.
I mean really, 1/2 or more of the 30-40 year old bikes we buy for a few $100 don't end up getting rebuilt and run just fine. That's after 4 DECADES of who knows what kind of break-in and subsequent abuse, neglect, redlining...
I think the difference between one break-in method and another, or not being concerned about it at all (i.e. riding the bike 'normally' from day 1) might mean the difference between needing to rebuild again 20 or 30 years down the road. If you're concerned about that far into the future, I'd suggest stopping and smelling the roses today instead and enjoy your bike
I rebuilt my XS engine with 0.5mm over new pistons/rings, fresh bore (obviously) and a head job (love those head jobs) and it's pumping out 150+ PSI 2000 miles later and I don't think I was particularly nice to it from the get-go. I rebuilt it and didn't even start the thing for 2+ years, and I didn't keep dripping oil into the cylinders or keep it in my house. It sat in my cold/damp/hot/dry garage for 2 full Canadian years, covered in junk, getting dusty....
Bike runs like a champ and I'm enjoying the crap out of it.