Yeah --- I'll bet I've been to 30 "run in's" and more than that - dyno runs... chassis and motors... and it's always been the same -- do your absolute best to set timing etc BEFORE fire up -- fire 'em off -- check the timing and start wingin' 'em to the moon.... The rings these days are lapped in by the time they've got all the pistons installed.... the bearings are all coated with break in lube... you've got break in oil with the right zinc and blah blah... So it's really just the cam/lifter that needs to seat... and that gets lubed by splash - thus the RPM's...
BTW -- this is ANOTHER reason we all love ROLLER CAMS!
The last two motors I've done --- the LS2 in Ricks '55 and the blown stroker in Rudy's were both fire 'em up and they idle... and you've got all the time in the world to check stuff out without the raucous rpms. With the LS"x" motors --- there isn't even anything to check out!! OMG - just twist the key and drive away!