I usually replace them. It doesn't happen very often, so the extra expense isn't a big deal.

There have been times where I had repeat flat incidents on the same bike within a short period of time. Once it was because of a defective new tire with a wire sliver protruding from the tire casing. Once it was due to worn out rim tape. When I have 3 flat tubes lying around, I buy a patch kit and take care of them.