Would it be o.k. to have 10 gauge wire ran for a 20 amp circuit?

Yes, in fact it is often done for long runs to reduce voltage drop.

It better be. I have at least 1/2 dozen of them, maybe even a 15 amp. I came by a coil of 10-2 with ground when I worked for a wire and cable company. I also recycled one that used to run to the AC to give the refrigerator its private circuit.

Of course, where I live, there is no building inspection. I must say, the #10 is harder to work with.