All of my experience with aircooled engines (airplanes and VW's) indicates that the dipstick will read noticeably higher when hot.

For consistency, I always check car engines hot, since I usually do it while getting gas.

With an airplane, you need to check it before start, hot or cold, for obvious safety reasons. Like others have said, the difference is not large maybe 1/4 - 1/2 quart depending on the engine. Most engines are supposed to be checked hot, and their dipsticks are calibrated accordingly. The main point of this is that if you overfill a cold engine, you waste the extra oil because the engine will consume it rapidly.