Coincido con los compañeros en que jamás he puesto dicha resistencia; es más, a veces incluso ni los condensadores. No obstante, googleando sobre el tema, he encontrado que una característica que define el comportamiento de los cristales es la ESR:
Equivalent series resistance (ESR)
(1) The resistive element, measured in ohms, of a crystal device. The ESR measurement is made only at the series resonant frequency (FS), not at some predetermined parallel resonant frequency (FL). The motional inductance (L1) and motional capacitance (C1) are of equal ohmic value but are exactly opposite in phase. The net result is that they cancel one another and only a resistance remains in the series leg of the above equivalent circuit. Crystal resistance measured at some parallel load resonant frequency is often called the "effective" resistance. (2) A term used to define and quantify the resistive element of a crystal.
Dicha ESR es menor en los AT strip cut crystals que en los AT cut crystals, y por eso se le suplementa con resistencia en serie.