Twisted Pair Twist Rates
Its actually a fairly complicated topic, I don't know of any "rule of thumb" calculation but there is an article from EDN here on transmission line parameters of twisted pair cables including the effects of pitch (twist rate).  The twist rate will change the effective dielectric constant between the 2 conductors which does impact impedance and dielectric losses vs frequency.

There are also other things going on.  For instance not many people know that the 4 twisted pairs in an ethernet cable all have slightly different twist rates.  This prevents the same conductors from constantly and periodically being next to each other in the cable which would cause increased non-common-mode interference.