Dunno how <bt> is selected, but it's definitely safe to say you won't know for sure which it'll hit.
As for <stpc>, I prefer to leave my main target (for <t>) on the current mob being fought. That way, all my normal enemy-target stuff can be <t>. Anything that I would anticipate using on some *other* enemy than my typical target, would be <stnpc>. Like Lullaby. Not much need for that on my main target, and if I do want to sleep my main target, it'll be defaulting to that anyway so I can just hit the macro and hit enter without retargeting.
But since I don't wanna hassle changing my main target around all the time (because that *would* quite likely lead to my screwing it up sooner or later), I want to be able to do things to players without changing my main target. So I use <stpc> for anything that involves targeting a player (such as any cure, status cure, targeted buffs like prelude ...).
If I were meleeing (which I do often enough in situations that aren't exp), and consequently were *forced* to have my main target be on the thing I'm whacking, <stpc> wouldn't just be a convenience to save me from changing targets, it'd be a necessity to target someone other than the mob.
If I were a PLD, sure, I'd *usually* be curing myself, so <me> could be understandable, but a good PLD will not only cure himself but will cure someone who steals hate and gets hit (you get hate for curing someone with hate, particularly if that cure actually heals for a sizable amount rather than 0), or will cure someone who needs curing when the PLD is personally at good health (same reason, more hate for curing for a real number than for curing for 0). So the PLD should have <stpc> too, because while decent-and-better PLDs can self-cure, the good PLDs can cure other people too, if that's a smarter choice (for party survival and/or holding hate) than curing themselves.