It seems you can do either - I often leave mine on it’s Qi charger 24/7 when at Home. Similarly I leave the Sonos Move on its charger-loop too and both Speakers and their battery have been fine.

If the Roam stays in one place it would make most sense to keep it plugged into the USB charge cable. I have a couple set up like that. 

Less energy wastage compared to a Qi pad, though I’d be the first to agree that during the UK winter the excess heat is in fact useful. In Mexico, perhaps less so.

Keeping Lithium based batteries fully charged results in maximum battery life. Repeated full discharge minimizes battery life.