It boils down to your comfort. (well, and the fast that you can posibly rattle bolts loose, but thats another topic)
I have poly mounts in my GP. The cam is ~kinda~ lopy, and makes the car rattle a bit, and then when driving and hitting a certain rpm it will get a vibration till I get about 1000rpm above it.
Its kind of annoying but, I deal with it, while some of my friends think its annoying as hell.