how do you keep your chain tensioned? EDIT - I think you may have just got lucky with your chain tension?
the other reason to keep the middle ring in it's normal (middle) position is that it probably won't fit on outer position - but it looks nicer on the outside and you just move the cog outwards by approx 4mm to match it in terms of chainline
the other reason to keep the middle ring in it's normal (middle) position is that it probably won't fit on outer position - but it looks nicer on the outside and you just move the cog outwards by approx 4mm to match it in terms of chainline