What's with the cavalier attitude on a DIY forum? Killing power to the 24VAC transformer doesn't hurt anything; and it's just good practice to disconnect anything electrical from its source before working on it.
Nothing cavilier about it,you can have the power company shut power off to the whole block if it makes you feel better!
Just saying its not a major deal to do it with the power on is all.
But by all means do it however you like!
"facts" have no relevance to this discusion
Posting from a concrete bunker under a non descript barn