I would guess the instructions so commonly given are due to the common mistakes new folks may make. Saying to use tap water is wrong as they may go off and leave the media in a bucket of chlorine treated water for a few hours which will kill a lot of BB. Say use treated tap water and they may use HOT water. So as a dodge for a number of ways to screw up the process, "old tank water" is one which will cover if they leave it for hours or any number of other ways to mess things up. Once you find what is required and what is going to harm you and your tank, you are likely to move some of the common advise a bit further back in your mind. Watch a bit of the tube advice and you can see all kinds of ways that things can be done---but not all are safe for the newer folks. Knowledge is power and once you achieve some of that knowledge, you can then begin to find shortcuts. But there are two ways to gain that knowledge. Killing your fish is just one of those ways so it is better to use the safer advise until you know the why and where of shortcuts.