Water heater warranty still valid without installation invoice?

Hey guys, fully licensed plumber here with 10+ years experience. So like many of you, I do the occasional water heater replacement for friends, sans invoice = cash. Just curious if you've ever had an issue dealing with a manufacturer warranty without an installation invoice? My supplier keeps my receipt for the tank on file, I'm just wondering if I could potentially run into an issue in the future if I need to warranty a friend's heater that I did for cash. Experiences?