I consider myself an upper intermediate player w/ a rating that fluctuates between 1700-2050, and I've gone through GM games where they move their bishop in front of their pawn on e6 in the Dragon variation of the Sicilian and on d3 in the English. Also I've seen them move their Knights on the rim usually a3 and a6. My question is when is this good and when is this bad?

Look at the game closer, you're probably going to find out!

Anyway, I figure putting a bishop in front of a pawn is ok if you don't intend to move the pawn in the near future and it controls important squares, and putting a knight on the rim can still be good if it can control or reach important squares from there, even if it controls fewer squares in general.