Generally speaking, 5 arcmin or better is pretty good. You can enter values into the calculator at
http://celestialwonders.com/tools/ro...ErrorCalc.html
For instance inputting an alignment error of 5 arcmin, FL 1000mm, Guide star angle 0.5 degrees, declination 80 degrees and exposure time of 5 minutes yields a field rotation of 1.6 microns which is much less than a pixel on most cameras.