I agree. I'm terrible at maths so struggled with aspects of Computer Vision in my degree, but I don't see how you could use OpenCV without understanding the principles to at least a basic degree. It seems like you're purposely creating a black box within which magic is happening. Which would be nice and fine and abstract in many circumstances, except OpenCV keeps you greatly abstracted from the concepts, and any non-trivial Vision application needs you to get close to the theoretical-metal anyway, in my experience.
If anyone's interested in the theory, I'd recommend Sonka
totally. while OpenCV is really useful and magical (i did learn a lot of theory from the OpenCV tutorials though), it didn't help me wrap my head around concepts, but implementing the algorithms (gaussian blur, edge detection, etc) without having fully grasped the math helped a whole lot with understanding how things work. i still can't explain the math behind edge detection, for example, but i can describe how it works. when i wrote this, i had in mind a person who would like to get a big picture idea of what image processing is, and will hopefully be inspired to learn what happens under the hood.
If anyone's interested in the theory, I'd recommend Sonka