Never lock my bags, and nothing has ever been taken (knock on wood). Depends on weighting though. Usually you can get away with a few extra kilos (I've done upto 5), and if you speak Chinese and beg/flirt around you can get lucky and they will lower your fee (I was 20kg over) a bit. My fee got reduced to 15-10 and if my Chinese was better I am sure I could have done much better. Being sociable to the check in girl always helps.