Ok guys thanks, the replies make good sense. My scenario has always been where I haven't yet developed the bishop as there are limted if any good places for it and I'm in a rush to develop rooks/castle etc.
As you all say though you have to weigh up pros and cons. No set rule. I seem to be going through a phase where I'm rushing myself into trading wanting an endgame type of mentality... 
@rogwilco
a specific example is the Austrian Attack of the Pirc:
1. e4 d6 2. d4 Nf6 3. Nc3 g6 4. f4 Bg7 5. Nf3 0-0 6. Bd3 Na6
Black wants to prepare ...c7-c5. Here white has the option of Bxa6 but won't remove his guard from the centre. The bishop g7 in combination with a R on b8 can pose trouble on the queenside. You'll never see a strong player play Bxa6 in such a position.
As always, as already noted, there are no hard and fast rules and it comes down to a judgement and analysis oof the specific perculiarities of the position
Bit of a weird question but lately I've been presented with this situation numerous times and have never been sure what is the correct way to move.
If playing white and you have not yet developed your bishops and your opponent moves their knight to either a6 or h6 and can only recapture with b or g pawns, should you simply trade? I'm thinking it's always better for white because one black has wasted a tempo moving the knight in the first place and two that black now has a doubled pawns? Also presuming that there is currently nowhere advantageous for the said bishop at that time. Is that sensible thinking or not?
Apologies if that post really isn't clear. To simplify the question, should you always trade if it results in the opponent receiving doubled pawns?