Well the issues I have seen with using em's or percentages in your body tag is that it can change according to the browser defaults.
Defaults are usually set to 16px within your browser preferences, but what happens when someone changes their browser default to say 10px, now your 62.5% now becomes what? 6.25px? and then all the em's for that page are now offset by that amount.
I have considered setting the high level font size to a pixel value then a child of that value can be used with em's. for example if you change your font-size in your html or body to say 10px
then anything inside it can be used with em's like wrapper{font-size: 1em} would equal 10px. So you start with a value you know, then use em's to scale accordingly. need 12px? go to 1.2em (i think that's the math
)
Although to be truthful in the matter, I usually just end up using px's as my value's, but I can see the benefit of scalability in a site if you set your sizes and margins, etc with em's. but you just got to remember that we do not have control over the user end, and that the user can change the starting value on us.