As I understand it, the working conditions for those involved in making reusable grocery bags is deplorable. Make sure yours are made in the USA.
Additionally, unwashed canvas bags might be a serious disease vector, due to the fact that multiple people slide their (unwashed?) hands in and out of your bags and other people's dirty bags - all day long. They need to be washed frequently to be safe.. which probably just shifts the environmental damage somewhere else. I don't use them for this reason. I'm hoping stores will switch to a new form of "bioplastic" bags made from plant starch (corn, potato) that can be composted without special treatment. The currently available versions, don't compost easily outside of a commercial composter.