Well you know what they say (Ass)umption makes an Ass out of you and I was certainly guilty of that here On-topic:
I have it at 256mb and I'm an Addon-Holic, never seen it go over 120mb in-game .
Maybe I'll need the breathing room later in large group content though, I'll think about increasing it more; system ram is abundant nowadays.
To OP: If it doesn't stick with LYQ's trick you could try setting the config.wtf file to read-only (right-click properties), you'll need to remember to revert that change if you want to changes settings at some point though.