The only thing I'll really lay down strict in that sense is that NA vs. EU duels must be fought on an East Coast server. Other than that, it's frankly just common sense. If a Westerner and Easterner fight, you're not going to have a duel on an NY server, but on a Central one. There's plenty of them around, from Robo's to Wappaw to a few others, so choosing a server should not be an issue. I realize many duelists feel a strong difference between 70 ping and say, 40 ping, but oftentimes a server can't be provided that's just for you. Anything below 75 ping in the case of an NA vs. NA match should be fine.
I realize that in the case of a Westerner vs. EU match the pings for both players would be awful, but that's honestly the only way for me to make the fights at least a little more fair for the euros.
If an issue does arise, however, it'll be taken on a case-by-case basis. I'm open to suggestions and what not, as I duel quite a bit but I'm not seriously competitive in the game mode.