Usually when I am out at the mall and DS needs to be fed I go to Nordstrom because they have a nice mother's room that has a comfy couch. If I am somewhere other than a mall I just find someplace a little out of the way and pull out my nursing cover.
I don't think it is a big deal to BF in the dressing room but I would be annoyed if I was waiting for a room and you were using it to BF in.
I BF in dressing rooms all the time. In fact, I BFed in the Gap last week. I have yet to actually NIP at a mall b/c it's easy to find a dressing room or do it in the car before/after shopping.I know several other people that do the same.
I don't ask unless there's aren't a ton of dressing rooms or they ask the # of things I'm trying on.
The only mall I ever go to has an awesome family lounge that has three private breast feeding stalls and many comfy arm chairs to use.
If I am in other stores, I just use my nursing cover.