My singlespeed mountain bike chainline is off about 6-7mm because of a seriously mismatched BB and crank (will fix this soon). Anyway, its still fairly quiet and I've never dropped the chain. Road bikes function well with the chainline off by several cm. You can't get away with that with a SS chain, but you can still get away with a lot of slop in your chainline. Nonetheless, I'd still aim for getting it to within 1-2 mm. If you are measuring your chainline with just a tape measure or even a digital calipers, you're measurement error is going to amount to 1-2 mm anyway, so don't expect it to be absolutely perfect.