Closed radiant heat systems will use metal pipe, or PEX with an oxygen barrier. This means that if working properly they will not rust inside (no oxygen). I don't see any purpose in draining it periodically, since all you drain out is the stale, somewhat dirty water. Its not like a water heater where you get a lot of sediment.

If the guy knew what he was doing he should have spent more time talking about pressurizing the expansion tank (a safety item) and periodically bleeding radiators (makes the system work better). He should have also showed you how to read the pressure gauge and water temp gauge(s) to make sure the system is operating at the right temp and pressure.

Bruce