I rebuilt an old-timey diesel once. It took 20 minutes to get oil to the rocker gear in all 6 heads. I'm glad I used assembly lube there. That's a little extreme, but it takes a good 30 seconds to get oil pressure to the head in my Yamaha TT225 and that's with a new oil pump. 30 seconds without lube is a long time, especially when you're talking about extreme valve spring pressures seen on MX 4-strokes.
Before I get attacked, the point of the story was to emphasize how restricted oil flow to the head can be and how long it can take to establish oil flow to the head after a rebuild. Of course, on our tiny engines we're talking seconds and not minutes, but we're also talking much greater pressures and a much greater potential for wear. Quite a bit of damage can be done in those few seconds.