I don't pay cash often, but when I have, I've pre-paid inside the store, and the pump automatically stops when I've gotten the correct amount of gas. It may be regional. This shows people posting a mix of pre- and post-pay anecdotes: http://www.democraticunderground.com/discuss/duboard.php?az=...
Edit: At least in Europe you usually pump before paying by card/cash in the store. I guess it differs from the US because of the high crime rate there.