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Hi Stuart,
No, that is not a problem.
The star images from a Newt with a parabolic mirror will generally alway only be point sources in the centre of the field of view and will begin to elongate as you look closer to the edge of the field of view in any direction, that is unless you have a field flattener such as a Paracorr. Obviously this will be more apparent with faster optics.
To some extent top quality eyepieces seem less effected by this.
For the same reason when you star test, the difraction rings should only be concentric when the star is in the centre of the field of view, both inside and outside of focus.
I use the Tectron set of collimating tools along with a Ezy Collimator double pass laser and occassionally check with a star test, however I have never found the star test to show a problem with my collimation.
The ultimate test of your collimation really revolves around the quality of the images you see at the eyepiece or a tougher test is when imaging the planets at very high powers with CCD's. If you are happy with the images you see through the eyepiece or if you are getting fine detail imaging at high power then there really is no problem with your collimation.
Regards
Trevor
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