I think the idea of "pixel perfect wireframes" is a bit of an oxy-moron. They are intended to be a structure (as someone pointed out when talking about sculpture and the origins of the word wireframe) and a tool in the process of getting to a pixel perfect design. It's better to step back and remember that they are an artifact and not the end design so use them in that way...to find the answers to your questions and as a communication vehicle and don't stress over the pixels!
I've talked a lot more about this in my recent article on UXmatters called "Sketches and Wireframes and Prototypes! Oh My! Creating Your Own Magical Wizard Experience". Also referenced the article Will Evans mentioned where he talked about some of these concepts as well.