The reason you should always coat aqueous-printed canvas is that all aqueous media receive ink into a "receptor coat" that is bonded to the surface of the media. The receptor coat has to be there in order to 'receive' the aqueous ink. The inclusion of the receptor coat is also why aqueous media costs more than latex/solvent media.
As some have said, you can get away without coating aqueous canvas prints, but if you do, that means for the life of that print, the receptor coat on that print is still going to be open, and receiving. Not a good idea if you're truly serious about your product.
And if you are truly serious, the best printer for printing canvas there has ever been is the Canon iPFx400 series. It's out of production now but if you want to try and find a used one, you can for not much money.
Next up quality-wise I'd go with the current aqueous Epson Sure Color.
If you don't want to go aqueous, the absolute hands-down canvas winner currently among commercial-grade latex/solvent printers is the Epson S80600. After that, the HP solvents actually print very well, but they do have serious gamut limitations compared to the Epson.
Mike Adams
Correct Color