  Recently computed terms of orders O(\alpha_s^4 n_f^2) in the perturbative
series for the tau decay rate, and similar (new) strange quark mass
corrections, are used to discuss the validity of various optimization schemes.
The results are then employed to arrive at improved predictions for the
complete terms order O(\alpha_s^4) and O(\alpha_s^5) in the massless limit as
well as for terms due to the strange quark mass. Phenomenological implications
are presented.
