all about chainline, if you're interested:
http://www.sheldonbrown.com/chainline.html

And FBinNY is right, 3mm further out won't be a practical problem at all. If you rarely use the chain in the outer sprocket, it may actually slightly improve your efficiency (but again, those efficiency issues are rarely important for avg riders anyway)

A forum like this one just tends to be populated with mechanics who care about small details.