Tue May 28, 2013 10:38 pm by MBI
It depends on the bitting of the lock. It doesn't HAVE to be all 9s, but they do that since some locks out there for that keyway will have a pin chamber or two with a 9-length pin in them.
A bump key with all 5s should work fine, IF the lock you're using it on doesn't have any cuts deeper than a 5 depth. Otherwise, if you used a bump key cut to 5s, if you had any stacks that were a 6 or deeper, you'd be over-bumping the chambers with the longer pins. You'd be bumping the top pins out of the way, but at the same time you'd be bumping the bottom pins IN the way of the shearline. To put it in picking terms, you'd be over setting that pin.
That's why some people prefer their bump keys cut to about a 9.5 depth instead of a 9 since it can tend to work better on locks that have a 9 depth on one of the pin stacks.