The whole issue of turning props backwards was discussed in
this thread around 6 weeks ago. The general consensus (from far better technical brains than me) was that for most modern engines, there will be no damage if the prop is turned backwards. There are, however, vaccuum pumps out there which will suffer from broken vanes if the prop is turned backwards and you need to refer to the POH for your aircraft to find whether there are any specific warnings about this.
Personally, I am in the "I don't turn the prop backwards unless I've got a damn good reason" brigade - saves remembering to check for each type I fly. This may be perpetuating an urban myth, but seems pointless to take the risk if it's not necessary.