Breathing Colors varnish works well, but is rather pricey. I print for an "Artiste" who loves the canvas, and uses the same Liquitex Varnish over the giclees that he normally would use on his original acrylics. He applies it prior to stretching.
We print his work on a Roland FJ500 with Lumichrome Inks (available from the Digital Imaging Group). Nice media combination.