It is not really a code issue but I would still bond a steel sink to the EGC of the counter receptacles via the copper piping. I couldn't cite a code section tho. I have a stainless counter top and sink in my kitchen and it is bonded to the cooktop EGC (no neutral load) and receptacle EGCs.
If you have a disposal it is probably bonding a steel sink.
... but I am a bonding freak. The steel in my driveway is bonded.
I think Sparky's answer is oversimplified. It's never wrong to protect them with conduit as long as a steel conduit is choked on both ends, but 250.64 (B) certainly says that there are situations where a grounding electrode conductor size 6 or larger may not require conduit as a means to physically protect it. Guys?