I had a draft coming in from the cabinet above my microwave where the exhaust fan vented through the wall to outside. The vent pipe going through the wall had gaps all around it and I used expanding foam for that without a problem...fixed it right up. I also had a draft coming from the bottom of my gas fireplace (where the millivolt valve and blower are located), there were a couple of small holes cut out of the sides for what looks like wires to be run if needed. I used some high temp great stuff foam for that, so if you're really worried you can use that type of foam.

I don't think there is much danger using foam under the sink, if you're like me you probably have cleaning supplies that are way more flammable than the foam.

Actually, just the opposite. The standard expanding foam has a high fire rating, thus only burns when a flame is on it.

If there is flame impinging on it, you have other problems.