Although I don’t know alot about it, I believe you can go into the IPO window, select RotZ from the IPO types to the right, select the curve, go to the frame that you want the rotation to end and put a key frame on the slope at that frame (Ctrl Left Mouse Button).
Then simply “flat line it” at the same value as the value at the key frame. In other words, end the slope. The way I do it is to add another keyframe for that slope at the end of my animation and make it’s value the same as the keyframe entered at the point where the wheel rotation ends. Exact values can be entered by going into Edit Mode, selecting the point, and pressing N. The value you want to enter is the Vertex Y value. This keeps the wheel(s) from rotating.
You might use a bezier curve (Select RotZ Ipo Type by left clicking RotZ from the choices on the right … select the slope by right clicking it … press Tab to go into Edit Mode, Select the key frame point where the wheel stops rotating, select Point from the header menu, then select Interpolation Mode -> Bezier. This will make your wheels gradually slow down at the end of their rotation. You might have to tweak the curve by selecting the control points and playing with them to get the result you want.
I’m just getting into animation as well, but I believe this might help you.