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I went to a Sur la Table a few times which was offering free knife sharpening.
I asked if you had to have bought the knives in the store, and they said no; it was basically just a way to get people into the store.
I didn't have my knives with me at the time, but would probably trust a place like that over the local grocery store.
Alternatively, just go to any farmer's market and there's always a guy operating out of his van who knows what he's doing, for $4-5 per knife.