What is the best way to maintain uniform font sizes on my website using the stylesheet.?

The reason I ask is that it seems if you make a modification here and there, you get different font sizes in different places. and it does not look uniform at all.

I know that there are different sizing methods (i.e. em, px, pt, %). I also know that w/ZC, right at the top of the standard stylesheet there is font-size: 62.5%; within the 'body' element.

With all this said is it best to use 'px' to keep the font size a fixed size vs em - if you need (or want) to make an adjustment? How does it all work?

Thank you.