The piston rings will create a better seal when the engine is at normal operating temperature, so a warm engine may see you some different compression readings.
The main thing is to be consistent in the way you check each cylinder, it's the difference between the readings of each cylinder that is of more importance rather than the readings themselves. For example 160 psi on all 4 cylinders would be fine or likewise 140 psi across all 4 cylinders would be still OK, but if you had 3 cylinders at 160 psi and one cylinder reading 140 psi you've got a problem.