My mother uses a wheelchair and when I took her shopping, I got sick of people talking to me when it was her that wanted the item and had the money to pay. In the end I'd wheel her to the queue and leave her there. She got treated like a human and I didn't get caught in the 'does she take sugar' rut.
I'll say something like 'alright there mate?' if I think they may require assistance, but I tend not to go right in for the 'I'll get that for you' because although they may need help, they don't always want help.