i had mine done at house of fraser. i wasnt intending to cause i hate the whole experience, but this scary italian woman accoseted me and before i knew it i was in the changing rooms with my top off. she didnt tell me to take off my bra, just looked disapprovingly at it and tugged at various points saying "look, no support at all. she only took my back measurement, no tape was used for the cup size, she just looked at my chest and said "yes, you are a dd, maybe even an e". i was confused cause i have been wearing a b cup for years, thats what the m&s tape measure said when i was 14 and i stuck to it. but she brought me some to try on and i put an e cup on and was like "by jove, it fits!". apparently bra kings rigby and peller dont use tapes at all, they are trained to look at you and work out the size that way, so in theory you shouldnt have to remove your bra. i have never been asked too, but a number of my friends have told me about their horrifying experiences when they first got measured with their mums and had to strip. glad it wasnt me! i think if they ask you to remove it they wont be offended of inconvenienced if you politely decline.