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?