Being off a bit does net matter except to anal retentive types. Sometimes a person will cob together a combination that is way off and biases the chainline one way or the other to favor large or small cogs.If one uses the BB that is recommended for the crank being used,it's usually ok and that's how its done with all Factory bikes or even shop assembled ones using stock components.One can play with chainline by using different spindle lengths or an adjustable BB. Playing with spindle length is difficult in cases where the shimano splined system gives you 1 choice for a double and 1 choice for a triple in road systems. current campy does not give much choice either, but at least with the ISO taper there are some aftermarket choices available.Shimming the BB is often another possibility for small changes on the drive with Cartridge BB.