A short time ago on Facebook @Ian Cooper wrote something that might be worth preserving & commenting here as well (also for those who are not on FB):
I don't know how & when YGMV came up, but I always detested it. I don't remember it coming from Greg himself, by the way. For me, it was always WILL & not MAY... as in Your Glorantha Will Inevitably Vary & Cannot Do Otherwise (YGWIVACDO)! Anyway, Ian really nails it & says it very, very, very nicely.