Best approach to add custom user profile fields in multisite?

Hi,

I want to add a few custom profile fields to profiles on a multisite network. Most of the multisite setups I've had in the past used Buddypress so I could just use extended profile fields. I also know I could add this to the theme's functions file on a typical wordpress instance. However, what is the best approach to this on a plain multisite network?

Thanks,
Nathan