Let's say, if the professor signs their email with first name, and then you keep calling them Dr. xyz or Professor xyz for a long time, you might come across as too distant (happened to me before). but I also don't really know what's the protocol: should I switch to first name? should I ask? Asking is a little too awkward for me -- in that case i'd rather be too distant than asking. Switching to first name might offend people, but then if they introduce themselves by their first name or sign their email by first name, and then I call them as how they refer themselves, can they really blame me???
If a prof signs their full name, of course you should be formal.
Ugh this is such a hot potato question