You should use a fast diode, rectifier diodes tend to have a decent amount of capacitance with their PN junction cause it needs to be large to pass a lot of power. When the diode is reverse biased that capacitance holds a charge, when the voltage goes forward it takes time for that charge to dissipate, during that point the voltage is building and fast. It's only important in this type of application because the voltage spike the solenoid switching creates is probably on the order of 200+ volts and it's nearly instantaneous. If you keep that conduction path open all you end up with is a small current spike through the coil in the reverse direction when it's switched off. The capacitor may help but it may not help enough, and keep in mind it has to be a non-polar cap. Your call on weather you want to try it that way or not, I wouldn't.