You don't have to, if you want to go with XHTML then I assume you basically want the "standards" behind it, the ones that are basically encouraged by W3.
Do what Google does, code your pages the best way possible with less tags, view your pages on multiple browsers (and versions, if possible). If all is well, then no need to switch to XHTML transitional or strict.
Though, the standards it offers is not bad, most websites that comply with the XHTML standards are well coded, well formed and generally has less code for relying on CSS to style everything rather than the usual <b><i><font> and so on.