I leave the sign on all the time from engine start to engine shutdown. Seems like the pax always disregard the sign and get up whenever they feel like it. This way I'm covered if they bump their head in turbulence. Of course I always make sure they are seated before takeoff or landing.
As far as checking on pax, for me, it varies. Depends on the impression I get of their demeanor(s) as they board, as well as the length of the trip and time of day.