Taking a deferred rest day

I was plain exhausted this morning and quickly decided not to run. I figured that since I only took one rest day after my race, I was entitled to another this week. It wasn't a hard decision to make. It's an opportunity cost scenario: What's worth more to a tired runner, an incremental run or another day to recover? I wouldn't say my guilt meter is at zero for skipping my workout.  But it's pretty low.