I agree that all caps in buttons is common practice. But compensating for ascenders and/or descenders is a bad idea as each font family is different by its x-height, ascenders, descenders, etc., etc. means that 100px “sans-serif” is 116pt and 100px “Source Sans Pro” is 106pt.
How compensation will be applied with changing font size dynamically. Even with fixed sizes per viewport it will be PIB. Once you will be done client will say that they like to have different fonts and you can start all over again.
In my experience, the pixel-perfect obsession comes from Graphic designers as on “paper” is possible to do anything but the majority lack of understanding HTML and CSS world. I was thinking to do a video about this topic but it will be too complex, which means long. There are many great sources about Typography and Web design.