"There is no reason to enrich your mixture during a descent. If you have a constant speed prop, it won't let you over-rev the engine to the point that the mixture is too lean."
I think that needs to be clarified a bit because with normally aspirated CSP, injected engines it may depend upon whether you are sitting behind a Lycoming or Continental. The Lycoming (RSA system) uses a Bendix fuel servo that provides a feedback loop based upon actual MAP, whereas the Continental system basically makes assumptions about throttle position and fuel flow rate based upon a mechanical linkage between the throttle and the injection pump. With the latter, if you make a powered descent from 9k to 3k at a constant RPM starting at cruise settings, especially if lean of peak, and do not touch your throttle or mixture settings during the descent, you will find yourself running increasingly over-square and lean as you come down, however if you adjust the throttle to maintain cruise MP or lower during the descent the engine won't know or care at what altitude it is. The issue is not that the CSP prevents the engine from over-revving, but rather that it causes the engine to pump a constant volume of air for a given manifold pressure. At the same time the continuous flow injection pump will deliver a constant volume of fuel at a given RPM and mixture setting without regard for actual MAP, meaning that as you descend without throttle adjustment the MAP will increase, the engine will pump more air for the same amount of fuel and the mixture will lean at the same time cylinder pressure is increasing, which is not desirable.
Anyway, the "lean vs rich" debate has been around as long as the single vs twin debate. For some, the rule may be, "If you own the plane, run rich, if you're just paying for the gas run lean", however systems vary in ways that can be significant, which is why one should always consult and follow the recommendations and procedures in the POH for the aircraft they are flying no matter how much more one believes he knows about the plane than those who built it.