There are two things that I would try.
1) Make the button graphic larger vertically so that it doesn't stretch.
By this I mean the size of the background without changing the size of the image. You can use magic pink for the background or transparency if png. But, I wouldn't make the button larger than the vertical size of the taskbar.
2) Set your margins so that only a acceptable area will stretch. For round buttons, I choose the one pixel in the center. For example if my image is 15 pixels high, I choose top and bottom sizing margins as 7. If your image doesn't line up properly, you can add magic pink across the bottom and then adjust your sizing margins accordingly.
