Based on my experience with my car, I have found that the most reliable reading comes when the engine is cold or when the car has been sitting for awhile. While it is possible to take a reading when the car is hot, I have not found it to be as accurate.
However, I believe one should try to take readings in multiple situations to get a good feel for what the dipstick is actually reflecting. That is the only way I came to my conclusion with respect to my car.
70 to engine rebuild faster than you can say "poofff!"