I can't recall ever noticing negative effects from dry hopping at cold temps. However, my new procedure is to cold crash before dry hopping (maybe even xfer to secondary) to minimize interaction between the yeast and the hops that could result in undesirable flavors.
I just can't believe you never had the grassy flavors from dry hopping cold. Really? I do agree that it is best to get as much yeast out of the beer as possible before dry hopping, because as the yeast drops out it grabs some of the yeast with it. But, without a doubt, dry hopping in the 60-70 degree range works best for me.