Hi Mike,
VMware sees it as a best practice to run on the latest hardware version (see the Virtual Machines section on this page: VMware KB: Upgrading from vSphere 5.x to vSphere 6.0 Best Practices) and if you run into a problem with a VM, support may ask you to upgrade the version. In practice it might give you some more stability or some performance improvements.
I have been similar to you though in that I don't necessarily keep my VMs up to date at the hardware level. I make sure they are relatively recent though.
You will want to make sure that all guests in the cluster are compatible on all hosts; the last thing you want is to have a failure and find out that the guest can't run on a different host due to an incompatibility.