In practice posting the cap throws the pen out of balance, especially kit pens because of the weight. The postable option is a selling point but most won’t bother because of the balance.
The clip also acts as roll stop.
Ballpoints don’t have a cap and therefore aren’t postable.
I think most high end mass produced pens such as Mont Blanc, Visconti etc are not postable, if they are then the cap slips over the finial rather than threading on. I’ve always thought this method could lead to scratching the body finial.
My kitless pens can be posted this way but will it mark the finial or damage the finish on the wood ones, probably. It could also damage the threads maybe.
All down to personal choice ….. and balance 
