Yes, I meant sanding with the grain. It is really not that much more time involved, I work slow anyway. It means more to my that when a pen leaves my shop I know that I have a quality pen that will last and the customer is happy.
The vast majority of customers won't know the difference. If it's about making the customers happy and saving time, skip the longitudinal sanding.
For
me - and from what you say, I think for
you - my goal is a little loftier: I want
me to be happy with the finish. And I find I'm happier with it if I do longitudinal sanding between every grit.
For
you, I say: experiment. Make two pens, one right after the other, from the same wood and same conditions except the sanding. Then do one one way, and do one the other, and compare the results under whatever lighting and magnification you care about. If one is unsatisfactory, you can quickly re-do it using the other process. Either way, you'll have your answer.