Thanks for all the comments - most insightful.
There is a practical reason why I’ve asked this. Whenever I initiate correspondence with a dive op who don’t know me, or when I post here and want to put what I say into the proper context, I often will say that I’m a beginner. I think that sets the right context for my opinions in SB and the dive ops will know not to expect too much of me in terms of skills (as an aside, I have a lot of diving knowledge which I got here, but not skills because I literally haven’t dived enough). But after say, 50 dives, if I still think I’m not “good enough”, can I still legitimately call myself a beginner or will people start thinking I’m full of BS?
I'm curious as to why you feel you need to describe yourself to a dive op when making initial inquiries. Most dive ops will do what they can to give you the kind of dives you're interested in and comfortable with. Instead of saying "I'm a beginner," maybe consider something more informative (to the dive op), such as "I prefer to avoid swim-throughs" or "I prefer to avoid dives over 90 feet," or even "I'm a bit of an air hog," etc. I don't know whether I'm a "beginner"--as other replies here suggest, it's a fairly meaningless word--but for Cozumel trips, for example, I tell the divemaster that my thing is marine life, and while I don't fear typical swim-throughs, they just don't excite me as much as seeing the animals. I try to give them a better idea of the kind of dives I am looking for.
As far as using "beginner" as a caveat to your opinions here, it seems to me that anyone who's interested in a poster's background can look up the poster's profile as well as previous posts to get an idea of who is behind the post.