I always found BA to be pretty good (if that's the word). They'd only turn the seatbelt sign on when it was getting pretty bumpy. On take off, they tend to turn the signs off once smooth air is encountered above the clouds.

Contrast that to Continental. The seatbelt sign is on until the cruise is reached (usually 30 minutes or so). As soon as there is a ripple, on it comes again. Maybe this is to do the 'compensation culture' that prevails in the US.

I was on one CO TATL flight where the seatbelt sign was not switched off for the whole journey. Of course, then everyone just ignores it, and the FA's are pretty powerless to do anything.

Cheers,
Rick