There is no acceptable or unacceptable difference, it's your car so only you can decide what you're happy with.
Personally I don't use the spring collars in this way. I balance the car using lead, and use the collars to remove any tweak. Since I also build the car without any tweak in the first place, the difference in collars will always be small, as it will only be accounting for manufacturing tolerance, difference in springs, and any small amount of tweak left from the build. I would say if my collars were more than 0.5mm apart I would start looking into why.
RC-Timing Software - http://www.rc-timing.com
Mercedes AMG F1 - http://www.mercedes-amg-f1.com/