The best temperature for Taq polymerase is 72C. If your primer Tm is higher than 72C, then the polymerase would not work well, since the primer will not anneal to the template. If your primer Tm is lower than 50C, there are chances you would have non-specific bands. Therefore, in my experience, Tm for primers can be 50 to 70C, the best range is between 55 to 65.
