What are the rules of thumb for margins in web design?

My web designer tells me that in a web page, the empty margins or padding should always be multiples of a standard. For example 6 px, 12px, 18px. This should produce nicely balanced lay-outs. I would like to learn a little bit more about it:
Should one really not violate this at all?
Should the standard be the same horizontally and vertically?
Anybody know some online tutorials or examples of good/bad?

Thanks in advance for your input!