Are there any specific guidelines in using CAPITAL LETTERS for HTML/PHP tags rather than lowercase? Is it okay (in terms of search engine optimization) to have both capital letters tags and lowercase letter tags?

Example:

</TABLE>
vs
</table>
If it doesn't matter, does someone have a recommendation or tool to go through hundreds of PHP/HTML pages and change all appropriate tags to lowercase or uppercase? Can DreamWeaver or Notepad++ or TextPad do such a thing (without screwing up any other text/source, etc)