Declaring an emergency seems to be influenced a lot by corp culture (and other cultural influences). From an unscientific sample using flightradar24, AF declares a lot, American airlines a moderate amount (considering their high traffic) AA the most, followed by UA and DL. The sample is also skewed by flightradar24 coverage areas...
Some airlines will declare an emergency, with trucks rolled etc, but never squawk 7700 - LY for example on several recent occasions at BGN.
It would be interesting for someone with more statistical knowledge than I to analyze the data from flightradar24 or another source. To be fair, the one time I declared I was already in the Class C, and I didn't change my squawk either.