IIRC I didn't finding any easy way of making xmodmap settings persist over reboot, so I just took had a script that runs "xmodmap ~/.Xmodmap" run at startup.
(Of course then I switched to xfce on my laptop, which seems to run its own xmodmap command after its run startup scripts or something which overrides them, which is kinda annoying, haven't found a nice way to get around that).