A lot depends on the client's experience. Someone who primarily writes with ball points will generally have a much different grip and writing position than will a fountain pen person. A fountain pen is best used with very little pressure to the nib, is generally cradled in the hand and gently guided rather than held tightly and forced into the paper. As such, balance is much more critical.
Most celluloid or resin based production fountain pens are made to be posted, have very thin, light bodies, and balance well posted or not, some feel unaturally light or unbalanced without the cap posted. I have a large vintage Parker Doufold, which is a tad larger than an El Grande, yet considerably lighter. The cap posts without disturbing the balance at all, yet I would never post the cap of an El Grande or Churchill, as the balance is too compromised.
The remaining commercial pens are either metal based, usually lacquered brass, or some combination of celluloid or resin with metal sections and sometimes metal finials. The former tend to be thin and heavy, most are top heavy when posted. Some of the more ornate, larger and more expensive commercial pens are not made to be posted, primarily for balance reasons.
My personal opinion is that none of the kit pens feel right when posted, all are too top heavy compared to a well designed commercial pen meant for actual writing (some are made primarily to sit in someones box and look good). The Jr series are reasonably well balanced un posted, but the long skinny section is a deal breaker for me. The full size Gent, Statesman and their derivatives are much too heavy for me, and the metal sections are a deal breaker. I made one once, was quite impressed until I actually tried to write more than a few sentences, at which point I sold it to a friend. He likes it, but he has no experience with other fountain pens. That leaves me with just El Grande of Churchill styles for my own use, or a few kitless pens that I've made of similar proportions. The kitless are lighter and more comfortable to use, and are not postable.
Dan