Its a bit like the slant that some people claim can be seen in knitting if you don't ply your yarn. If its over or underspun, regardless of whether its s or z, you will probably have problems using the yarn, particularly if the twist has not been set. I know some spinners say they never set their yarn, and have never needed to, and/or wind off onto a niddy-noddy immediately the bobbin is full, etc and others treat that as heresy.
I accidentally wound off a bobbin the other week and there is a difference between that skein and the others of the same yarn that sat on the bobbin and rested over night, although I'm hoping that will not be so obvious if I decide to ply the yarn. I also tend to err on the side of caution, and set the twist, even if I am going to ply the yarn at a later (or sooner) date.