metric 0 vs 'no metric at all'

Hiya, I was wondering if someone had done any or some research on this before... basically I am not sure with all the many implementations of BGP and all the vendors if and what those will do when they see a metric of 0 and no metric. I am not an expert knowing the actual protocol's messages exchanged, but I see some routes with nothing in the metric field on the various show commands, and some have explicit '0' metric. I do not trust all the BGP implementations around, and we consider changing the default outbound, with MEDs of course still available on request. Alexander