I can understand that. There is still something in the back of my head though.......that nagging feeling! I mean I have a Newbery SPS handpicked, I also play with decent Dukes balls. You always worry about what might happen I guess.
Would also depend on where you bat. If you train with old balls, and you don't open or bat top 4, then you should be facing an older ball a fair bit and that won't affect the bat as much as a new one would. So you could get away with a bit less knocking in if you bat middle order.
I am having a bat prepared atm by KD sports over here in Oz, which I pick up tomorrow, and I'm pretty sure they knock the bat in by machine, so I can let you know what it's like when I get it. I'm sure others would have experience with a machine knocking in service though.