Don't know if it's a requirement or not, but I was taught the same thing in the Czech Republic . It's been a few years, but I seem to remember it was more about practicality than anything else:
1. You can see line and other features more easily from the LH seat if they are on your left; and
2. If everyone does the same thing, there is less likelihood of conflict with opposing traffic.
I have to say though that this (requirement / suggestion - delete as appropriate) seems less well observed since the mass introduction of GPS into the GA cockpit
