As an ATCO, I would want a 7700 squawk ASAP, especially in an emergency descent. I would also like it for all other emergencies that require me to give an a/c priority.
It's not just the aircrews workload that increses during an emergency - an ATCOs does as well. Squawking 7700 tells everyone else around to get out of your way and thus helps reduce some of the workload.
I don't see any reason why 7700 cannot be selected at or near outset of an emergency the majority of times, including emergency descent (apart from maybe an explosive decompression). It can be done immediately after the 'immediate actions', i.e. at the time that you should be transmitting your call to ATC.
The words 'emergency descent' make the manoeuvre sound more punchy than it actually is - it should still be a very controlled manoeuvre, albeit slightly uncomfortable when you put the masks on etc.