I'm with Don, sorry. The ultimate test of optics is an in-focus star. If that's good, there's no need to worry about anything else.
Actually, this is not a very meaningful test unless you have a known perfect sample to compare too and even then, it fails to tell you very much about a variety of errors. It is useful for astigmatism and comatic curve in a mirror but little else. A comatic hill in the objective might make a small bulge in the ring, and Astigmatism could make several with the cross being the most severe kind, but this is not an error that we see much of.
The eye is not very good at judging small differences in brightness. Suppose there is some spherical (SA) aberration in the system (and this is the most common "severe" error in most reflectors and compound scopes.). SA will make the first ring brighter, but in reflectors and compound scopes, that ring is already pretty bright because of the diffraction cased by the secondary mirror. If there there is 1/6th wave of SA, the ring, already being quite bright from the obstruction, will not appear so much brighter that it is easy to see unless you have another perfect instrument to compare it to and even then, it may be difficult to see. The eye is just not a very sensitive radiometer when it comes to brightness and a star is a very high contrast object, so the subtle glow from SA or other errors is not easy to perceive because even if the background brightness is raised by 5%, this is not really enough to jump out at you.
Next, zonal errors, rough optics, and a turned edge are all very difficult to see on an in focus image. These errors will draw light from the spurious disk and spread it out around the star, but unless the errors are quite severe, it can be difficult to see this because even the normal sky glow can hide it unless you are under extremely dark skies. That damage will be there though, and on a planet, every point on the planet will scatter that light around that point lowering contrast.
Seeing an in focus star with a round first diffraction ring does tell you that there is no coma or major astigmitism, but it leaves you clueless about these other errors, all which are easy to see and estimate severity using an out of focus star.
(People used to day double stars were a good test, but this is even more useless. In the presence of SA, the spurious disk actually gets smaller and this can make a scope with meaningful SA split closer doubles than a scope with no SA, thought the chances of finding a double that would be "just right" to show this are rather small, but the point is that even with meaningful SA, a scope can still split most doubles that the perfect scope could split. The exception would be if you compared two otherwise identical scopes but one with SA on a "Just right" double, the brighter first diffraction of the scope with severe SA could make a star that sits just under the maxima of the first ring harder to see in the SA scope than in the perfect scope, but once again, this is a Goldie Locks kind of chance to find just the right star in either of these cases.
I don't want to discourage anyone from doing whatever they are most comfortable with, but an in focus star is not really telling you much about the overall precision of the optics. Even a so-so telescope can produce nice diffraction pattern. If you are happy with your scope and use if mostly for just looking at diffraction rings, then that is all that matters I suppose. If though, you want to really know about the quality of the optics, more advanced tests are going to tell you a lot of things that the in-focus star test won't tell you.
Makes me happy LOL 
