I always try and avoid the till points with fund raising bag packers at them but sometimes they all have them.
In Asda the other day there were 3 early teen girls standing at the end of the conveyor belt. I politely declined their offer of help but they then sort of leaned over the packing area, jostling each other and talking/giggling loudly. One seemed to be staring at me most of the time! Today in Sainsbury's it was the scouts - a couple of boys who looked about 10/11 who were being silly and pushing each other.
I don't want to risk them squashing stuff and I object to feeling pressured into giving them money. I usually do tho, because by the time I've packed my bags myself I'm starting to feel guilty (which annoys me!) so i drop something in their bucket.
I suppose it must work but personally I'm inclined to be more generous with people collecting for charity who aren't so much in your face.