If the manufacturer says it doesn't need draining, then you should take their advice. Sounds a bit iffy to me?
All I'll say is bare in mind that the name "flow switch" indicates that something is flowing (ie water) in order to activate / make the switch. If its on the ch side, the pump induces the flow of water to make the switch when you turn on the heating. If its on your hot water side, the water running through the boiler when you open a tap makes the switch.
I'd drain it if I were you.
Baz (CORGI Eng)
May 2007