It's all down to the engine.
In Lycoming engines the carb is usually located close to, if not attached to, the oil sump. Therefore a lot of heat is automatically transferred reducing the overall risk of ice. Students are still generally taught to liberally apply carb heat, and in a lot of schools to use it for every approach. This is completely unnecessary with a Lycoming engine, but for the average PPL it's probably good to get into a habit which will work on every aircraft, and there are engines out there that will simply stop if you reduce power without carb heat. For more on use of carb heat see this thread
http://www.pprune.org/flying-instruc...carb-heat.html
With older continental or rolls royce engines, the carb was positioned further infront of the engine leaving it much more exposed. I believe this was also the case on some older Gipsy engines found in Chipmunks and the like, but my experience on these types is fairly limited.
The main area of risk is from 5 - 20 degrees C, and the big danger is that it will happen in clear air if it is humid enough.
The only time I've had serious carb ice with a Lyco is taxiing on wet grass with a typical OAT of +8 to +15. Ultimately you have to know your engine and it's own dangers.