Good point about x height. Font metrics can vary greatly depending on the typeface you are working with. This is the same regardless if using web fonts or not.
I include a font previewer in my WordPress theme options panel.
This allows the site owner to preview the font as well as its metrics in context of adjacent in-flow text blocks. I also include a custom css box to allow them to adjust margins, padding and line height as needed for each individual element in which fonts are applied to (Site Title, Site Tagline, Post Excerpt and All Hx level headings).
Hope this helps someone. Google web fonts, when implemented correctly, are brain dead simple and rock solid across every browser I've tested them on.
There are over 3,000 individuals & businesses who use my WordPress theme, representing tens of thousands of websites, and I've not received one negative support issue on Web fonts.