I did mine at just under 120K. I would suggest that you go ahead and replace the idler pulleys while you are doing the timing belt. I can't recall the source of it, but I was under the impression that it is wise to replace the idler pulleys every other time you replace the belt (approx 120k). The main thing for me is: better to be safe than sorry.
p.s. - this probably is more of a Technical forum post.
"The two seater is being joined in a few weeks by a larger two-plus-two model with a rear seat for munchkins." ~Jim Mateja, Chicago Tribune, in regards to the Z32.
Andrew Janeshek // jnshk@aol.com // 1992 NA 2+2 [Stage 2-ish]