Idle temps are never a worry. It's the temps you get when gaming, rendering, encoding, or whatever CPU-stressing stuff you do every day that are a concern.
If all you do is surf the Web and work on some Word documents, you shouldn't ever have to worry about temps just because the CPU never gets loaded doing those things.
As far as case airflow is concerned, all you have to do is make sure that there aren't any hot spots that don't get any air at all. Make sure all of the major components get air -- CPU area, memory, hard drive(s), graphics card(s), etc. CD/DVD drives don't really count. It doesn't have to be hurricane-force winds either -- just a light breeze should be enough. And of course, make sure that warm air flowing over the major components goes out of the case through the top or rear.