There’s nothing worse than getting in the dressing room, stripping down to your undies and finding out that the pants you want to try on aren’t going to fit. You can avoid the whole scenario of getting redressed, finding the rack they were on, getting a new size and going back just to find your fitting room is now taken, if you just grab a couple different sizes with you when you go in the first time.
Of course, if you have your outspoken friend there with you, you can always send her to get a new size. But, really do you want to stand there and stare at every dimple and crevice until she returns five minutes later? I don’t think so.