I find those easy days, where I'm just to tired to ride hard are ideal to just spin- low gear/ high cadence.
The result? Lots of quality miles/kilometers where I work hard, finish not too tired and don't find myself burnt out with sore muscles.
These miles/kms are the basis of a better style and being more relaxed on the bike. If you don't believe then try it - do say 10ml/km at 110rpm in a small gear and see how you feel next day. Post back and let us know.

So if you feel like an easy day just take it- you deserve it!