Is the TC ("terminal count") output of a 4-bit sync counter (74163) reasonably assumed to be glitch-free?
I'm assuming that you are referring to RCO, the ripple-carry output, rather than TCO.

No, you can not assume that it is glitch-free. In fact, the Texas Instruments datasheet says 


  counting spikes may occur on the (RCO) ripple carry output