Obviously, mirror slap is worse with telephoto lenses. First, check your self timer. On some bodies, when you activate the self timer, the mirror flips up right away. When the photograph is actually taken several seconds later, the mirror vibrations have died away. Otherwise, you need to conduct some tests.
Some bodies are much worse than others.
George Lepp has reported results on several cameras in his newsletter. You can easily test you own cameras to identify the shutter speeds to avoid. Very slow speeds are not a problem, because the mirror vibrations are only a small percentage of the total exposure time. At any rate, complain bitterly to camera reps every chance you get. For qualitative results, a good loupe and light box are all you need.