All of the advice so far about margins should be taken to refer to page/screen level tags (e.g. <body>, ...), other that the good advise to not use the shorthand notation.
Margins applied to Heading tags (e.g. <h1>, ...) and containers such as <div> and <p> are another matter. With these, margin-top and margin-bottom are excellent way to insert vertical space, if you use em units, though added space is best (IMO) left to special cases with <p> and <div> tags (eg. <p class="section" where the section class has a margin-top of perhaps 1em).