If I'm approaching people on foot I go with "behind you" because it's a simple message and they're just gonna do whatever they do anyway. I slow down and go whichever way they don't, which is usually me going left but not always. If I'm approaching someone on a bike (very very rare up here in the sticks) I'll go with "on your left", on the assumption that they know what that means and because the V-sub-C is low enough that they'll have time to process and react in some sane way.
(In the surrounding 144 square miles I've seen people on bikes eight times, four of which were probably the same guy and two of which were probably the same woman. Plenty of people walking on the greenway though.)