While there may be guidelines as to whether to climb or descend when you're icing at your current level, individual pilots that I have known varied widely in the specifics of what they would do depending on their experience, their comfort level in icing, the route they were flying, and their experience with that route in bad weather. My take follows.
First, @acpilot's comment gives the general guidelines and is worthy of being an answer, I believe, except that I would be happy with just a couple of degrees above freezing rather than 5°C.
I put in a few hundred hours in Turbo 210s back in the 1970s. The ones I flew had boots on the wings and hot props. As such they were a reasonable machine to handle the icing of the U.S. Pacific Northwest. The things I wanted to know to decide whether to climb or descend included:
- what's the rate of the icing
- what kind of ice is it, rime ice (the most prevalent) or clear ice (basically freezing rain)
- what kind of cloud am I in, is there convective activity
- how high can I expect the icing to persist, are there pireps giving the tops
- how much distance will I have to fly to either be in the clear, between layers, or get to where the minimum safe altitude is above freezing
- what's my climb capability, in other words how heavy am I and how much has my climb capability already been degraded
Even with knowing those things, it's still as much a subjective decision as an objective decision, at least it was for me.
The distinction between rime ice and clear ice is particularly important. Clear ice (basically freezing rain) is the more dangerous, accumulating faster than rime typically and heavier. It always (almost) means there's air greater than 0°C above you, which would favor climbing.
Rime ice is minute drops of super-cooled water (water less than 0°C) that changes to ice when it hits the aircraft. More often than not, rime ice means there's no air above greater than 0°C, unless you happen to be in an inversion. Climbing out of rime icing usually means you've got to get high enough to get clear of clouds or get the temp down to -15°C or so.
Fortunately most icing is rime ice rather than clear ice. The rime usually collects slowly, although it can collect quickly in convective conditions. I once iced up with rime ice in a Cessna 340 in less than a minute to the point that I was having trouble maintaining altitude.
Much of my T210 flying in bad weather was between Eugene, Oregon and airports in California's central valley (shown above left to right). V23 was the airway from the Eugene VOR to the Medford VOR to the Fort Jones VOR to the Red Bluff VOR and then south into the central valley. The airports were typically above freezing, but the MEAs were in the ice if the weather was bad, and you were guaranteed to get at least some rime ice. The MEA from Medford to Red Bluff was 10,000, and that was where the worst ice was usually encountered, but the MSA dropped rapidly south of Fort Jones.
If southbound from Eugene and approaching Medford you had already picked up more ice than you liked, and the Medford airport was above freezing, you could shoot an approach there and drop the ice before proceeding south of the Siskiyou Mountains. Once the OAT gets above freezing, the ice melts really fast, typically on the order of a minute or less. The windshield clears first. Importantly, for rime ice, the rough surface of the ice smooths quickly, permitting better airflow.
If it appeared icing along V23 was not going to be flyable, we would file over to the coast and go down over the beach so to speak with the attendant low MEAs and more airports, all usually well above freezing.
My point is, a lot of things enter into whether you should climb, descend, go another route, or get on a jet.
A couple of good (IMHO) links (pdf's) to icing information are: