I can't think that this can be all that critical and is probably, at best, a matter of taste. If you think about it, your bike has to work effectively on every ride with a potential difference in fork length at any one time of 80 or 100mm between fully extended and fully compressed and not spit you off into the trees, so it will hardly notice a one size change in nominal fork length. Extra length fully extended will tend to make the bike more stable. The minimum length fully compressed, at which point your bike is least stable, will not change at all, however much extra fork travel you add!
The static ride height difference between 80mm and 100mm forks will only be around 15mm, because they should be set up with the same proportion of sag. Mountain bikes come with steering head angles between about 67 and 72 degrees and bottom bracket heights that vary between about 11.5 inches and 13.5 inches. A difference of 15mm in effective fork length will keep it well within this 'normal' range and will barely be noticeable, unless the bike was already at one of the extremes of geometry - which your Trek won't be - it will be pretty average.
My Trek came with adjustable travel (80mm - 130mm) Recon 351 forks on it and different models in the range using the same frame have either 80mm or 100mm forks. 130mm does feels a bit like a chopper, but it is still perfectly rideable. There's not really any noticeable difference between 80 and 100.
Hope this helps.