Shouldn't the question be: Does specifying the font size
prevent the app/device from changing it?? If so, then it should not be defined in the CSS. e.g. p{font-size:1em}.
However, the way I understand it, setting font-size in ems only means that element's font is
relative to the selected display font size. e.g. font-size:1.2em in a header would display 20% larger than the display's selected font size and a 1em for the basic paragraph would
be the selected font size. In that case specifying p{font-size:1em} is not wrong, or even detrimental, it is simply being overly specific.
I would prefer someone being overly specific just to be on the safe side.
What I hate to see is when publishers define the basic font size as too small... p {font-size: .8em}