What is the preferred way to call chmod from a procmailrc?

:h
|chmod g+w /var/mail/mailbox
would unnecessarily feed the mail message to the chmod command,

DUMMYVARIABLE=`chmod g+w /var/mail/mailbox`
would create an unnecessary variable just to run chmod.

Is there a third way to run a command from a procmailrc?

Also: it might turn out that g+w is not enough and I have to grant o+w on all mailboxes. What is your opinion about this? You may be cruel, since I want to know the truth.

Oops! After a thorough revision, this turned out to be a difficult issue:

I have to call chmod from procmailrc AFTER the actual delivery receipt.
If I call it before, and the mailbox is empty, then nothing will be chmoded and procmail will give the mailbox the default permissions of 600 - disregarding any umask setting (which is my experience).

So, is it possible to call a command from a procmailrc file after, or as a last element of a delivery recipe?

Also, if I do not want o+w on the mailboxes, then I have to chgroup them to grant group 'email' write permissions to all of them (then g+w is sufficient). Presently the mailboxes are owned by their users and the users' default login groups.
Does it involve any side effect if I chgoup the mailboxes to an other group?