Hi,
We often leave the boat on the beach while grabbing some lunch and drinks. I think it depends on the wind as to how you leave the boat. If it's not pumping, and the boat is stable pointing up wind (say 15knots and less) its not a problem. Just make sure everything is uncleated, and the sails can flap easily. A sneaky one is the main sheets getting caught on the back corner. This way, with it pointing up wind, released down haul, it should be happy. Also, ensure it's compltely off the water. This way it shouldn't go for a solo trip
Just check on the boat in the conditions you want to leave it in. If it's rocking a lot, then rather drop the sails. Safety first. Don't want the mast to knock some poor sun bather out. Also, our club is on the beach, so the boats are only 30-40m away, which allows us to watch them. If you can't see your boat while not there, and the wind is dodgy, then rather play it safe.
But, in light days, pointed up wind, with no downhaul and the boat is stable, I wouldn't worry about dropping any of the sails.
Clint