One of the first things we do when optimizing a site is to institute common Best Practices. As important as best practices are, they are still only a baseline for good usability. They are really no more than starting point.
Once you have instituted your best practices and have tracked them in analytics for a while, you now have a place to begin making additional, incremental improvements. This is where multivariate and A/B testing comes into play. Best practices only tell you what works for the masses. Your visitors may react differently, and with some additional and ongoing tests, you may be able improve the overall performance of your site.