Assuming you have 120 ninjitsu and 100 hiding the difference between 80 stealth and 120 stealth is 34 point deathstrikes compared to 40 point deathstrikes.
Higher stealth also lets you stealth in nonmedable armor (Leggings of Bane?)
If you have 40 points to blow then sure run stealth up to 120, but its hard to know wheter or not it is worth it without some idea of what skill your are taking the 40 points from.