Bad practice and the main, but far from only one, is :
If you choose witch CSS to load server side, you'll not be able to handle the switch on tablet between vertical and horizontal mode (hz = desktop view; vt = mobile one), as you'll not have a reload of the page.
This is why you should load both media query in your CSS at the same time.
If you want to make the same behaviour, use this :
<link rel="stylesheet" media="screen and (max-width: 768px)" href="mobile.css" />
<link rel="stylesheet" media="screen and (min-width: 768px)" href="pc.css" />