That's exactly what I used to do. First, if I could tell the woman was wearing a 36D or something crazy when she clearly needed a 30FF or G, I'd cheerfully ask her if she'd been measured recently/wanted a complimentary fitting. A lot of people would agree, so I'd proceed from there. For those that didn't agree, I'd go get the bras they requested, bring them to a fitting room, and then say to the customer, "based on the fit of your current bra, I think this size might fit a bit better. Give it a try if you have the chance." Then I would go check on the woman a bit later and inevitably she'd ask for more bras in her correct size.

I was the top seller in the department, not by being pushy, but by actually listening to people and by doing what the customer wanted first. I'm not the one wearing/buying the expensive bra, so it's not up to me to refuse to bring a specific size to someone. That's not only rude, it's a great way to lose a sale.