Board Thread:Show-Related Questions and Answers/@comment-25315804-20180414042149/@comment-6302057-20180424150127

Snow abandoned the name Mary Margaret in 5x13: "I was someone who took risks even when she was afraid. I was someone who, I guess, inspired people. [...] I don't want to be Mary Margaret anymore. I want to be Snow White again."

Rumple/Gold was called by both names both during season 1 (by Regina) and after the curse broke.

Mother Superior was called Blue once in 2x02, with most characters favouring Blue from 3x11 onward; S6 almost exclusively used Blue with MS only used a handful of times. Belle and David are known to use a different one in different episodes.

With cases in SB post-curse, it really comes down to personal preference of the character themselves, and those that use the character's name in conversation.

As for Hyperion Heights, I guess it's easier to keep using the curse names even when alone so that they don't get too used to their real names and accidentally slip-up in conversation with characters under the curse, or risk being overheard. S7 characters currently use their real names when it matters most.