Basically it depends on who's hiring you and consequently the audience of your work.
For institutional purposes (where content should prevail over the shape, like a goverment project), you better work with .em or %, they are harder to control, but they will be really user friendly in terms of accessibility.
If we talk about corporative Websites (Where shape is the deal) pixel will be a more accurate tool to fulfill your customer expectation regarding his brand.
A liquid interface (%, .em) is always a good stuff when it is smartly done, but don't forget to check your design under extreme conditions and be sure that it will be stable.
If you work with pixels, you will have absolute control over the final look of your site, but you will have to assume the impossibility for some user's to operate with it efficiently.
Best option: rather than designing a Website compatible for all platforms (what will result in a multi-deficiency design) suggest your customers special versions of the site adapted for every demand, what is a better practice and a better business for a designer too...
