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You warm the oil not only to aid in draining it, but also to get the maximum amount of dirt to drain with it.
With the engine running, the oil holds alot of the accumulated dirt and combustion by-products in suspension.
When the engine is stopped, the oil stops flowing, cools, and allows much of this dirt and gunk to settle out of suspension.
The main purpose of an oil change is to remove the dirt. So, running the engine normally (not idling as this produces even more combustion by-products) allows the oil to pick-up most of this dirt. The sooner you can stand draining it, the less gunk will have dropped out of suspension and be left behind. Wear gloves or such, but change it as soon as you are able after shutting the warm engine off.
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