Remember inheritence. A tag will inherit most settings of its parents (like font-family). So, if an attribute needs changing, change it at the highest possible level. That would prevent to do it in child relations that are in use.
It sounds more complicated than it really is though. Also remember that you can combine styles (e.g. <p class="letter centered">). In that case all the styles will be applied. Usually in the sequence of calling, but I remember a discussion on the board earlier that some applications or readers use the order in the stylesheet.