ive rolled hundreds and never trimmed a one, now if your gun has tight tolernaces like Benelli youll need to trim but an american made pump like an 870 your fine not trimming, so it really depends on your gun.
guy asked how to you know when to stop the roll crimp, when everything is tight, if you can shake your shell and still hear pellets rattling you need to roll her on down some more. the big deal with roll crimping is getting everything tight to have consistent pressure from shell to shell.