Merlin,
Florida is hot and we drink a lot of water here. I haven't heard of that convention or rule.
I was taught to always step laterally out of the line if needing to drink...and then drop to the back if the line moves up. Aside from keeping pace, one just doesn't have the braking control of the bike with one hand when drinking. That said, I don't always comply and sometimes drink when in line but don't when pulling as a rule.