I have Yoshimo in my party and he has leveled up twice and i now have got him at 100% picking locks. My question is the next time i level up, should I go beyond that to like 110-115(traps too) or should i distribute them to finding illusions etc.

Finding Illusions is the theifs most usless abilities, so put the points to either hide in shadows, lockpicks, or pick pockets.

You'd better invest in another skills, like Detect Traps or Pickpocket. 100 for Disarm Traps is enough. Similarly, 100 for Detect Traps is enough. But 100 for Pickpocket is not enough.

<font color="gold">Yeah, I agree with Radek here. YOu should use many points on Pickpocket, and also Detect and Disarm Traps. ;) </font>

In order of importance:
Traps: 100%. The Ring of Danger Sense is quite good, but the Planar Sphere is too difficult to do early.
Locks: 75% is enough to start with, but get the Ring of Lock Picks as soon as you can.
Hide in Shadows: about 60% in the beginning, depending on how much you want to scout/Backstab.
Move Silently: Don't put anything into it at the start, but about 5% per level should be enough to keep you Stealthed long enough to get the job done.
Pick Pockets: You'll be using Potions to augment this when you shoplift, so don't put anything into it at the start. Later on, try to get up to a natural 100%, and the Gloves of Pick Pocketing will almost ensure your safety when you're out working the crowd.
Set Traps: Every battle can be won without them, so it's safe to leave them on the back burner for a while. Go ahead and set your Traps anyway, the worst that can happen is your Thief will need a Cure Medium Wounds.
Detect Illusions: Leave this one for last, but it's anything but useless. At 100%, it's like your Thief has True Sight--I'm not kidding! Of course, it's only active when your Thief is Detecting Traps--go to the "Customize Script" page and check out the "Thief Adventurer" script.

Did you finally settle on a guitar yet, Ozzman?

Da best skil is hide in shadow(!!backstab!!)

everyone is afraid of setting traps and getting damaged by them. not if you have enough points invested into trap setting. 100 points into trap setting 100 into trap disarming 100 into hide in shadows and 270 (max)into thieving. just remember traps arent always the grestest in alot of situations but remeber this most bosses allow for you to set up 7 of those little buggers up around them. makes for easy boss kills