I do it anyway
Whether the seat belt sign is off or on...........
Havign experienced CAT over the Pacific and seen my partner heading for the ceiling (quick reflexes prevented a head smack) I always have my seat belt buckled when I'm sitting in my seat. I may have it reallly loose to accomodate the beer-storage facility and to give me room to squirm around, but it isnt uncomfortable at all and it will sure as heck stop me from hitting the overhead bins.
Just my own little preventative measure.