I painted all my rebar with primer and rust-preventive paint using a special technique I came up with. A piece of 3/4" diameter PVC pipe is bent to make a paint reservoir. The rebar is pushed in one side, passes through the paint, and is scraped of excess paint as it exits by a conical silicone rubber tip attached to the end of the pipe. The tip is formed over the conical end of a new cartridge of silicone and is trimmed with a knife to get the appropriate size hole.
Rust can cause the rebar to expand and damage the cement, so for best quality work it is best to paint the rebar.