We have a rather complex site that includes a Members Only

We have a rather complex site that includes a Members Only section, which is tied to our organization's member database. We have used an outside IT consultant to connect our membership database to our Members Only area. Every time there is a Wordpress upgrade we have to run it by our consultant, which tests it to make sure it does not break our customizations. Therefore, there is considerable cost to us every time we upgrade Wordpress. So, my question to you is whether every Wordpress upgrade is necessary and how I determine whether I have to upgrade or whether I can skip some upgrades to save money. Thanks for any advice you can provide.