I think this is a topic loaded with personal partiality. My answer is like usual, it depends. But first I'd like to answer to some of the things here I think are myths. I collect and restore a lot of vintage pens mainly from the 1930s through 1960s and predominantly of European origin (Italy, UK, Germany). My sample size of US pens is considerably smaller, so I can't be sure that my observations can be extended to the other side of the big pond without being very careful.
much of this debate ingores a very important issue which is that the further back in time we go, the less owners were concerned about the appearance or potential damage to their f.ps. - so they posted with gay abandon, and without the slightest thought as to consequences of damage to the pen. The cost of a good pen is now out of proportion to values of similar pens back mid C20, and replacing Vacs and Duofolds didn't cost what they might do now.
For the vast majority of writers pre 1950/60, a f.p. was simply an object of utilitarian necessity, since ball points were unreliable anyway, and a f.p. was seen as something essential in most occupations. That attitude is no longer, and folk who now use f.ps. are either collectors or people who write but not out of the same necessity.
Some of the terminal barrel constriction damage seen on f.ps. made pre 1950 is staggering - it's almost as though posting was achieved with a mallet, and many older pens are now damaged irretrievably due solely to posting. In view of cap rings, experience appears to show far more damage occurs to barrels than caps.
If you're worried about pens rolling, then use an appropriate holder on the desk, which will add some elegance.
I don't know how to phrase this carefully enough but I think this is pretty much the opposite of what I've seen so far. First of all, fountain pens, though a sheer necessity for the better educated, were not cheap at all. For the period of my interest as mentioned above, a good pen cost about the gross income of a week, which nowadays would put it up into the luxury segment. And therefore, people were not sloppy with their pens. But they also were commodities and thus not necessarily kept pristine for the showcase.
Second, posting a cap regularly over an extended period of time leaves cap marks on the barrel. This is unavoidable and one could live with it. However, most of the vintage pens I buy don't show significant cap mark, almost regardless of how small the pens are. And the average vintage pen in my collections is about 12 cm long when capped. From that I conclude that our ancestors did not have the habit to post every small pen because they considered them too short or too unbalanced. And honestly, with most of these nimble pens I can write unposted without any problem. It's a matter of getting used to it or what you expect.
Ritual and Reverence, i.e. when I sit to write it is almost always at my desk where I am now in the habit of immediately slowing down all my movements the instant I sit. If the pen I select requires posting to achieve the right balance then that procedure is carried out very carefully and deliberately, with gentle micro-adjustments before I start to write. I believe it is a combination of excessive speed and excessive force that leads to damage to the cap or barrel. In some cases, the design of the pen means that even careful posting will lead eventually to a posting ring on the barrel and so I must either use such pens unposted regardless of the balance or not use them at all if I care about the damage. #1 case in point is the Wahl Full GF Coronet where a posting ring significantly reduces the pen's value.
This is probably the way to go if you feel that you absolutely have to post your pen. This is close to how I do it. Sometimes I do post a small pen for what I consider better balance because, after all, I want to enjoy the writing. This is why I use fountain pens! But when I post, I do it very carefully for two reasons, first not to crack the cap lip if it's not protected by a proper ring and second, not to leave a mark on the barrel. This said, I do it only for extended writing session to make it worth it. Since I use my pens everyday in the office as well, there are many occasions when I only need to note down something quickly. In that case I basically never post because it would be rather inefficient and also could be harmful to the pen.
Anyway, since you are new to vintage pens I only can recommend to stay open-minded. It's another world - and there are no rules. But from my own experience I'd suggest to first try a new vintage pen unposted and learn its characteristics. If you really need the extra length or weight in the back then you still can switch to posting the pen. In the latter case you must be aware that it can damage the cap lip and leave marks on the pen's barrel. I do have a few Parker Vacumatics from the 40s with cracked cap lips most likely due to posting. I have an Eversharp Skyline with posting marks and in that case I'd say "fair enough, live with it because it's unavoidable". But most of my pens are free of such deficits and are still used mostly unposted or very carefully posted.