Do we need to check our sites on Firefox 3.5 and 3.6 both for
If my xhtml and css is valid.
If i upgrade my 3.5 to 3.6 then will i have to check site with 3.5 also. or both will render in same way. if 3.6 render differently then i would not upgrade because 3.6 has not much userbase as for now it will take time.
If your site is written with valid xhtml and css, there will be little-to-no difference from 3.5 to 3.6. Things like sIFR will only continue to work properly if their developers adhered to strict development rules as well, which they generally do.
If you're still concerned, consult the release notes.
For any new release of browsers, be it Firefox or others, minor or major revisions, it's always a good practice for web developers/designers to read the release notes.
Firefox/Mozilla has a dedicated site for developers for this as well. Check out https://developer.mozilla.org/En/Firefox_3.6_for_developers
Based on what you have described, there's shouldn't be any issue, but don't take my words for it :)