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Here is what I have always been told:

Compression should be the same warmed up or cold. If it is less when the motor is warmed up the it means that you may have a worn set of rings. This is because when things are cold they shrink up and seal better however, once warm the gaps open up.

Every compression test my mechanic has ever done on my motors has been while the motor is cold (air temps in the 70 - 80 F range).