Cautios nature gives +3 PE if you have an option ''do you want to encounter: Vedaras Own'' message. If it doesn't appear, you don't get your +3 PE.
No, it should always be used in determination of position at encounter, simply sometimes you can get bad roll (or whatever is it) and appear infront of critters.
My HtHer with 1 PE can appear in 1 hex within critters and near exit grid and don't even see them.
My main with 8 PE can appear in 1 hex within critters and near exit grid.
I don't see any difference, probably with 8 PE you'll have more chance, I don't know, but all last encounters with 8 PE I spawned infront of critters or between 2 groups of critters at centre of map far far away from exit grid.
So this 7 PE gap between 1 and 8 is almost nothing, of couse +3 PE by the perk will do double nothing.