Firstly, note that Find Traps and Set Traps are distinct skills. Find Traps benefits from numbers over 100, but only in a very few places. It's actually a threshold, rather than a percent-chance, so there's nothing magical about 100 in Find Traps. Most traps can be disarmed with an 85, but a few need 100 or higher. I'd recommend keeping a couple potions of perception around for those, rather than wasting points for a very small number of traps.
Set Traps, on the other hand, is a percentage, and is never penalized. As such, it doesn't benefit from numbers above 100.
@Naten This stuff gets pretty tricky because the actual mechanism for the thief skills is not apparent to those who just play the game. As a volunteer beta tester for the enhanced editions I have looked into the engine many times and I have a pretty good idea as to how things work, but even to me there is some stuff that is hidden.
That said, the thing you are wondering about I am certain about. The only time you need a thief skill over 100 points is if a penalty is applied to said thief skill. Aside from penalties from armor (such as elf chain and hide armor) the only thief skills that regularly get penalties are pick pockets, hide in shadows, and move silently. Hide armor and elf chain will apply a penalty to pick pockets, hide in shadows, and move silently as well as open locks and find traps. To my knowledge these are the only skills that ever get penalized and thus the only skills you would need to raise above 100.
The thing I am not so sure on is the critical failure and which skills have that, but it seems like you are not as curious about that.
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Set Traps, on the other hand, is a percentage, and is never penalized. As such, it doesn't benefit from numbers above 100.
That said, the thing you are wondering about I am certain about. The only time you need a thief skill over 100 points is if a penalty is applied to said thief skill. Aside from penalties from armor (such as elf chain and hide armor) the only thief skills that regularly get penalties are pick pockets, hide in shadows, and move silently. Hide armor and elf chain will apply a penalty to pick pockets, hide in shadows, and move silently as well as open locks and find traps. To my knowledge these are the only skills that ever get penalized and thus the only skills you would need to raise above 100.
The thing I am not so sure on is the critical failure and which skills have that, but it seems like you are not as curious about that.