Can't beat cold air ! I remember taking off from Big Bear (~6700') in minus 15°C, and the SEP, three up leapt into the air
We did have to find a hole to come back down through to get into the LA basin though (as BB was above the IMC), else we would have iced up in no time....
Generally speaking, if the OAT is above freezing at your level, you won't encounter icing. If you do, you must change level straight away, now whether this means climbing or descending depends on the air mass. Some instances (ie. warm front passing through) you may even be better off climbing if you start to ice (to get into the warmer air). The things that ensure icing are visible moisture (cloud, precip) and below freezing temps (though if extremely cold, you may not get any icing).
EA