  We study continuity properties on modulation spaces for
$\tau$-pseudodifferential operators with symbols $a$ in Wiener amalgam spaces.
We obtain boundedness results for $\tau \in (0,1)$ whereas, in the end-points
$\tau=0$ and $\tau=1$, the corresponding operators are in general unbounded.
Furthermore, for $\tau \in (0,1)$, we exhibit a function of $\tau$ which is an
upper bound for the operator norm.
  The continuity properties of $\tau$-pseudodifferential operators, for any
$\tau\in [0,1]$, with symbols $a$ in modulation spaces are well known. Here we
find an upper bound for the operator norm which does not depend on the
parameter $\tau \in [0,1]$, as expected. Key ingredients are uniform continuity
estimates for $\tau$-Wigner distributions.
