I'm quite sure it's not true for all, but maybe some. I've changed a lot of little boys over the 20 years I've been a nanny or babysitter and have yet to see an erection pee, actually.
I would suggest, however, and this is just something I've always done to avoid messes- when changing the diaper, first put a clean diaper underneath the hiney and in the right position to place on the child. That way you just slide the old one out and if there was to be a urination accident, you just put the new one straight on top, right away after pulling the old one out. (Obviously doesn't work to place it back on quickly after pulling the old out if you haven't wiped any poopy off, but it works for just pee-pees.)