Every time I'm fresh on strike, so start of an over or after a single, and always make sure fielders have moved back after I've got a boundary. Got to keep on top of it but no point being paranoid about a fielder going freelance on you!
I open and favour the leg side, so every time I make runs someone gets moved into midwicket sooner or later, whether deep or on the single. If on the single I ignore fielders, you're far more likely to miss them than hit them! If someone gets stuck out at deep square or deep midwicket after I've planted a few there then I'm either worried about clearing them or not. If not, no worries! If it's a long boundary and I'm worried about clearing it then I'll just aim for 4s/2s rather than lofting it, simples. To be honest though, usually if you lump a few to the boundary, yes their skipper will move someone there, but also the bowler will often learn and change his line so you don't have to worry about it much anyway!