Ye shall see the light my friend... mibbies

We get this quite a lot, after fitting new front tyres a car can pull to one side or the steering wheel can be slightly off centre.
If your old tyres were worn on one edge (normally the inside edge) it can be a sign that the tracking is out, even although the car didn't pull or anything, once the new tyres get fitted problems like yours can show up, this is because the car is trying to wear the tyres down on one edge again.
Just because the car appears to drive in a straight line doesn't mean the tracking is ok.... and just because the garage checked your tracking and told you it is ok, doesn't mean it actually is... their guages might need calibrated or perhaps the Toe settings are fine but the the rear flags (these are the guages on the rear wheels... these should point to the same number indicating that the car is centred, in other words, the front wheels are in a direct line with the rear wheels) could be out, if the front wheels aren't in a direct line with the rears, then the steering wheel will be off centre.
I'm not saying this is the case with your car, what you could try is swapping the front wheels from left to right (providing you haven't fitted directional tyres) do this to see if the steering wheels sits centre or the other way (to the right in your case) if so then it could be down to the natural track of the tyres, the natural track of a tyre is what way (left or right) the tyre will turn if rolled in a straight line in lab conditions, if it goes left.. the tyre is heavier on the left side... just like a bowling ball, if it goes right... the opposite. You will have noticed the lines on the tread of new tyres, one of these lines represent which side of the tyre is heavier, the fitter should fit the tyres so that the natural track of the tyres doesn't cause the car to drift to one side etc... I have also known this to affect the steering wheel. So swapping things around a little can aleviate any problems.
If after doing this nothing changes it tends to rule out a tyre problem, then If it was me I'd want the tracking checked again (possibly at a different garage just incase the original garage needs the guages calibrated) I would also get the suspension checked for worn bushings etc, althouh I'd think yours should be ok as it isn't a major issue you are having.
My money would be on the tracking being slightly out, but "still within tolerance", this means that the tracking can be out by say 1.5mm and still be classed as ok, I can't recall what the settings are supposed to be for the Yaris, but lets just say it's supposed to be -1... the settings could read...
-1 + or - 1.5, this would mean that -2 can be classed as ok. Like I said though, I can't recall what the Yaris settings are, so perhaps your is out, but still in tollerance... this could be causing the steering wheel to be off centre, but tracking ok.
If your not sure what I mean.. let me know and I'll ry to explain a little easier.