Just one thing about round stars greg i guide on cigar shaped stars very successfully
here is a photo of the type of stars i guide on and a result of a 15 minute capture do remember this isn't a G11 ro PME/X only a EQ6. I have always noticed as long as you have a clearly defined Centroid or a bright patch that is regular in shape then it doesn't matter what shape your guiding on. This is my experience and my images speak for themself as getting a EQ6 to track a for 15-20 minutes accuately isn't easy.
One thing that i have also found that really effects guide quality is your actual calibration, in maxim it moves the star left right up down if it doesn't produce a nigh on perfect " L " ill keep trying till it does. Things that affect this L is wind, polar alignment and balance.