There will always be a tiny bit of variability, but in general a 9 or 8 speed cassette installed on a hub mounted to 135mm dropouts will have a chainline at around 47.5mm. This doesn't vary from bike to bike, but may vary a little with different hub and cassette combos.
Front derailleurs generally have a working range of 47.5 to 50.0mm. You can get away with a touch wider, but you run into trouble going narrower -- especially if you're mounted to a fat 34.9mm seat tube.
Generally 47.5 is considered an ideal chainline, but you may go wider for a few reasons:
- Fat aluminum chainstays or suspension links cause crankarm clearance problems
- Short chainstays cause the chain to rub on the big ring, when shifted to the middle ring and smallest cog
- Fat seat tubes cause difficulty shifting into the granny ring
- 73mm bottom bracket shells cause the crank arm to rub the BB cartridge
What you can garner from this is that a 50mm chainline is acceptable for many reasons, and therefore is acceptable if you meet none of those criteria.
Be mindful of the actual chainline of gear combinations you commonly use. If you regularly find your self in the mid ring - big cog combo, every millimeter you bump the crank out results in that much more wear and tear on your chainring, cassette and chain.
Based on this, I'd attempt to set it up as narrow as you can go while maintaining necessary clearances and shifting effectiveness.
