I find that renewal rates are often more expensive than transferring your domains to another registrar (even with coupon codes and all). For example, you can use a renewal discount coupon but the private whois will no longer be free. But if you transfer out, the competing registrar offers free private whois only for new transfers + coupon codes. So it's cheaper transferring. Sometimes, some registrar suddenly offers a ridiculous promo like 4.99 for transfers, so if some of my domains are already eligible for transfers, i transfer them to that promo.
So i learned to just rotate my domains around the various registrars every year.