Should I put grease under the lockring on my cassette? I take my cassette off the bike to give it a good cleaning a few times a year and have always put a good amount of grease under the lockring. I am not talking about repacking the hubs, bearings or anything like that, just some grease to fill the gap between the lockring and the sealed bearings that are deeper down inside the freehub.

I've seen some references to greasing the threads on the lockring, but nothing about grease under the lockring. Without the grease does dirt/rocks/grass/mud get into that gap?
If the bearings are sealed (so dirt can't get into the bearing), and there isn't an interface (two surfaces rubbing together), than lube is probably not needed.  Adding lube may actually create a dirt-trap as more dirt sticks rather than just getting in there and falling back out.

The threads, certainly.  Packing in behind the lock ring?  Not so much.