As a guardian, do I need permission to use the ward's SS check?

You are conservator for this reason. So yes, you can spend the SS for their needs, whatever it is. The only time you can't spend it is if Medicaid is paying for longterm care in a facility. Then all SS and pension goes towards their care. But then Medicaid pays for meds and insurance.

If the person has any money of their own, a guardian should never be out of pocket for expenses. Guardian doesn't mean you support the person. Your responsibilities are to make decisions because they can't maybe because the other person is a minor or mentally incapable. Handling their money to make sure its spent on their needs.

Permission from whom? From the conservator?

I'm afraid I don't know the answer to your question - I'm sure others will - but it occurs to me that ideally you and the conservator will be working hand-in-hand and communicating on these matters anyway, don't you think? Is there some reason why this isn't happening?