Most reloaders I nwo don't trim rimless brass. However, I'm not one of those and have been maintaining all brass to within SAAMI spec., regardless of type. When I started relaoding several decades ago I read and followed the books to the letter and have been doing it this way ever since.
My take on why I feel it is necessary to trim 9mm in particular is because,
A. it head space on the mouth,
B. if I were to apply a crimp to remove the belling I would want all the cases to have the same degree of taper crimp, and this is a head space related reason.
There are several head space issues that can result if 9mm or other rimless cartridges aren't kept to within SAAMI spec.. I hear a lot of reloaders say that straight walled cases don't stretch beyond SAAMI spec., but in my experience i commonly see once and twice fired brass exceeding the allowable maximum as defined by SAAMI. In this regard I can honestly say that I have yet to experience a single mis-fire or other failure with any cartridge I've reloaded.
GS