Living in vancouver, for a few years only 4 minutes from the border, I've crossed hundreds of times. I always travel with my dog. I have never once been asked to provide paper work or been asked anything about her. HOWEVER, you are required to have their vaccination record with you. Including rabies. (see link below). I keep my originals in the car at all times. If I lost them I can always get another copy from my vet. The last thing you want is your "best friend" being denied entry while you're miles from home or worse not being allowed reentry.
If researching ignore anything that is not on a government websites. Regulations on both sides change frequently. I used to import animals as part of my career and have found contradictory information from different branches of government from both the US and Canadian sites. Border guards aren't always up to speed. (how can you expect them to be considering volume and changes). Word of advise…Don't correct them.
Pet food is one that has changed to allowing small amounts in original packaging traveling with the animal if you can prove the origin of the product. Despite this I have still had dog food confiscated due to the officer not being up to speed.
Various fruits/veg and beef/chicken are restricted depending on origin and where you plan to travel.
Easiest thing to do. Do all of your personal pet and human food shopping once you've crossed. Not only will this ensure you won't loose anything it'll make for a happier border guard when you answer "No" to the question and you will be far more likely to be waved though.
Sorry this is long winded but one important quote for a Gov. of Canada site. I'd think this applies for traveling north or south.
"Even prescription drugs and syringes used for legitimate medical purposes come under intense scrutiny. Carrying medicines in their original containers and carrying a duplicate of your original prescription, listing both the generic and trade names of the drug, is recommended..... In general, personal importation of a 90-day supply of medication is allowed, but only if the drug is not available in the United States. For further information, please visit the Web site of the U.S. Food and Drug Administration."
Canadian Food Inspection Agency - Importing Domestic Dogs
Import Policies - Pet Food - Animals - Canadian Food Inspection Agency
After all of that, I think you'll find it pretty easy. Someone will ask you where you're from, where your going, something personal and wave you through. They themselves may or may not be polite