Idle temps aren't as important as load temps IMO. Having a GPU that runs hot (heat rises and I've found heat from a GPU at idle can effect CPU temps a few degrees), or case fans can have such a huge impact on idle temp. Once your load temps are getting close to 60 you should start worrying, but I do like to see idle temps closer to 30C. Going overboard on a CPU cooler isn't a bad thing, my CPU idles around 20C and usually won't higher than 35C. But it's also cold this time of year too.