Some people say it's fine, period.
I know of no one who would say categorically that "it's fine" without some caveats.
I don't know what the "official" support stance is from VMware, but I will give you some real-world advice and also a personal recommendation.
Mixing builds within the same release is mostly ok with there being some serious caveats:
- feature sets change
- security implications may differ
- advanced settings are shifted with some introduced
Mixed clusters across different releases (i.e., a cluster containing 6.0 and 6.5 hosts) is a mistake and should absolutely be avoided. Caveats are as above but more severe.
My personal recommendation: Always, always strive to keep build numbers consistent within a cluster unless you have an extremely compelling reason. Laziness or not wanting to update your clusters is not a good reason, btw. Clusters across releases should be strictly avoided.
So what you need to ask yourself is this: Why am I wanting to have a heterogeneous cluster? You need a very good reason.