We do something similar, but from my experience small changes, like fonts or lines rendering a tad different after library changes can be quite frequent. Usually small changes you can't really see, only if you compare them as 2 layers in paint.net or something.

Adding something like an error margin for all pixels or subsections sometimes makes sense, but this can be tricky. Downscaling the image and comparing grayscale values with a small error margin is another option. It all depends on how accurate your tests have to be.