I always find builtin fonts difficult to pair when you want to use monospace for code. If you take Times New Roman and Courier New for example, Courier New is quite larger visually, so you would need to use a smaller point size for it. But how to do that when you need to handle fallbacks? On my blog, I am pairing Merriweather with a custom version of Iosevka to match x-height and ascendant height. I think this is less disturbing when reading.