SN74AVC2T45: sn74avc2t45 unused output ports tied to ground?

In theory, this applies only to inputs. Footnote 1 on p. 5 mentions only inputs; section 9.2.1.2 on p. 16 is clearer:

Unused data inputs must not be floating, as this can cause excessive internal leakage on the input CMOS structure. Make sure to tie any unused input and output ports directly to ground.

So in practice, output ports should be tied to the same level because they can become input ports when DIR is switched.

However, in a unidirectional application, DIR does not change, and outputs can never become inputs, so this would not be necessary.
(If you have a unidirectional application, consider using a unidirectional level shifter, but this is not always possible.)