I'm not an expert on the various types of vacuum pump, but remember reading an article with regard to this. Many vacuum pumps have vanes which are extended due to centrifugal force and are designed to rotate in one direction only. The suggestion was that by turning the engine backwards you risk turning the vacuum pump the wrong way and catching the vanes on the casing and damaging them. I don't know whether this applies to your particular vacuum pump.
If it does, better a crooked-looking prop on shutdown than a new vacuum pump!
