I have changed the default font of the jQ theme options from “Sans-Serif” (which was mainly Helvetica) to “Serif” (which is Georgia). While I much prefer serif fonts – and especially Georgia – I’ve always had a sans-serif font in the default styles of my themes because I believe that most people rather like fonts such as these. This used to be Verdana and later Helvetica.
As you can see, this is not only a question of personal liking but also of technical concern regarding font sizes. And there’s one more thing important to me when styling texts: the line height, adjustable with the CSS property line-height. I usually use a value of 2.0. This is the double default value! I think that makes text much more comfortable to be read. And it also looks better since it gives free space to the appearance of a page.