i've never really heard of the tensioner going out, it works off of oil pressure, though after about 200k i'd make sure its still within spec i guess. if it aint broke...

nissan never did a real recall on the upper timing chain's upper guide, but i think there was a TSB. i'd get it out sooner than later, i've seen many pictures of people finding theirs already broken, or cracked, and i have heard of a few upper guides being the cause of some nasty damages. the upper chain's side guide is also optional *i think*, according to recent readings on freshalloy. it's less of a problem than the upper guide, but i want to say i remember reading the side guide is something you can run without, but i'll check.

when i bought my car in '01, i think i had the valve cover off within the 1st 6months to check for the upper guide and replace the gasket anyways. my car had 60k miles on it, and the upper guide was gone, so i assume it was taken into the dealer. i even hear some of the late 97-98 model's didnt even come with the upper guides, though there are still holes in the head where it would be.

and it takes less than 15 minutes to do the whole thing start to finish.
-pull valve cover
-pull upper guide (2 12mm bolts)
-replace valve cover gaskets (surround and plug o-rings) (optional step)
-replace valve cover.

shizzam!