From: east coast, usa
Yes, not taking any action will get the best results. BUT ... the readiness level recovers at or less than the supply level. In some scenarios this is a long process, others are fairly reasonable. To get a grip on your recovery rates in a scenario, let a unit sit out of harms way for a few turns and monitor what happens (if enemy air, naval or artillery bombards it, it will lose some recovery). This is where those supply numbers do come in handy, because if you are sitting idle in a hex that has '9' supply, you probably aren't going to recover more than that per turn. In some scenarios its possible to hop on a train to a rear area that is in good supply (note that train embarked units do not recover anything while embarked).
Scenarios differ. For example, in Barbarossa '41, your German units will never recover anything so don't even worry about it, just keep plugging along with units in the red.
(Mmmm, I can taste that free beer!)