In 2/3/4 gang box, should you combine all grounds using crimp connectors?
You have to connect a ground wire to every device's ground terminal, that's not optional.  There's lots of ways to do it.  

You could crimp pigtails in the crimp ring, as long as you don't exceed the capacity of the crimp ring.  

One good way to go is leave one ground wire very long, and pass that through the crimp ring and hit all the ground terminals on the devices with that one long wire.  

If you don't have a long one, you could crimp in one long pigtail, and hit all the ground terminals with that one long pigtail.