As of 2.2 b5, Rawr.Bear is showing a level 83 mob as having the same chance to parry as a level 80 mob.
I suspect other people have done more in depth testing, but I tested against a level 80 target dummy on my 68 paladin first with 22 expertise (5.5%), then with 24 expertise (6%)
(link). I got a parry during the first test, then made over 900 hits without a parry or dodge during the second test. I know that expertise will give smaller increments of parry/dodge reduction than the tooltip shows, but I'm not sure exactly what the rating conversion is at 68, and I don't have the gear to get closer to 6% in either direction anyway.
I also tested against a level 80 dummy on my 80 druid with 72 expertise rating (~4.7% with Primal Precision) and 89 expertise rating (5.2%)
(link). Again, parry in the first test, then none in the second one.
Seems like the parry chance for an 82 mob is 6%, 81 would be 5.5%, and 80 is 5%.