General best practices for activating plugins?

So, I've been happily cranking along, adding sites to my two multi-site installations, installing plugins, and making everything work. First, I should clarify that I'm the only user for the sites (for the foreseeable future, anyway), so I'm not concerned about issues related to premium usage, etc.

Anyway, I've been downloading plugins and installing them on the Network interface, and in some cases, I've activated the plugins network wide and in others, I've gone to the site itself and activated plugins there.

Having gone back and looked at my sites, I now have a random mix of plugins for some sites that aren't used, and some that are.

So, is there a good best practice for this? Should plugins on a multisite install always be activated network-wide? Should they be activated on a per-site basis instead? What's the right rule of thumb for clean environments and good maintainability?