Generally, wc3 validation isn't critical, unless your HTML is horribly broke. Google is built to be able to handle all kinds of HTML that isn't wc3 compliant. That said, there are some things that will trip up Googlebot, according to a comment posted by a Googler on GGWH (can't remember the exact thread, sorry). So I'd say validate your HTML until you know enough HTML to not need to bother validating anymore.