Just thinking about it I'd say whatever plug runs well normally would be ok in colder weather. I'd think the most important thing would be getting the engine to (and keeping it at) operating temperature. Of course you'll have to adjust your mixtures. IIRC colder air is more dense -has more oxygen- so the engine would tend to run lean. I've also read that it's best just not to run once the air temp is getting near freezing or below.

<disclaimer>I'm by no means a glow engine expert</disclaimer>