You really should. If you only have one table on a page, there won't be any interaction problems. However, if you use images in your table cells, you can run into trouble. Same deal if you have more than one table on a page. You should use them.
James Garacci writes:
You must have them if you use tables in XSL style sheets with XML, always a good idea to close every tag with an end tag. Although to close <BR>, <HR> and <IMG> tags you should really use: <BR />, <HR /> and <IMG />