You'll have to seperate between using it to uncomment html tags and to hide scripts/css data.
HTML:It's better style to put them on a seperate line, but it's valid to use them everywhere. Remember that comments are markups, so don't place them inside another markup. Also avoid using a string of hypons ("---") inside a comment.
Commenting scripts in JavaScript:The JavaScript engine allows the string "<!--" to occur at the start of a SCRIPT element, and ignores further characters until the end of the line. JavaScript interprets "//" as starting a comment extending to the end of the current line. This is needed to hide the string "-->" from the JavaScript parser.
CSS tags:Use the following form to hide content of the style attribute for non-conforming user agents:
<STYLE type="text/css"><br /><!--<br /> H1 { color: red }<br /> P { color: blue}<br /> --><br /></STYLE>