So far as I am aware all of our newer templates are 3.5 compatible and therefore should be 3.6 compatible as a result. The same applies to our extensions.
As I ALWAYS tell clients - BEFORE you upgrade take a backup so you can get back to where you were before things went wrong. Ideally use that backup as a clone site to do the upgrade and test that everything works as expected. Then you can update the live site or replace the live site with the updated clone. This decision will depend on whether or not your site has data changing regularly that you need to keep e.g. ecommerce purchases, memberships, forums etc.
As a matter of good practice you should always update a template if we release a new version. The new version will contain bug fixes and also any changes for Joomla changes etc.
Hope this helps
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