In the design guidelines for Windows Phone, you are encouraged to use the font Segoe WP for the most of your UIs. Segoe WP is a sans-serif typeface.
Is there a convention or guideline for which serif font to use?
From the list of built-in fonts in Windows Phone, I can find two candidates: Georgia and Times New Roman.
I am aware of that using serif font on screen is considered bad by many people. But if my designer told me to use the default serif font of the platform, which should it be?