Eldritch Smite

Rules As Written

The Eldritch Smite invocation only works with Warlock pact magic spell slots. A multiclassed character cannot use slots from any other class.

House Rule

All spell slots are interchangeable for smiting (and for any other purpose).

Rationalization/Reason for the Rule

The PHB goes to great lengths to ensure that spell slots, once calculated, are interchangeable. Further, the Paladin's divine smite (which functions in an almost identical fashion to Eldritch Smite) can explicitly use any spell slot. There is no reason to gimp a Warlock arbitrarily, particularly since they have to burn an invocation to use this feature. When questioned, Crawford essentially said this was not put in for any balance reason, so it's gone.