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Programmatically 'disavowing' your editorial choices to link out to other websites isn't a great position to be in, in my opinion.
I seem to remember that Google indicated that NoFollowing every outbound link would indicate PR hoarding and would be frowned upon.
Doesn't seem to be doing Wikipedia any harm. (You can argue that's UGC - but it still indicates that there's currently no significant penalty for NoFollowing all your outbound links).