There is no evidence currently that COVID-19 is transmitted by food. The biggest risk when it comes to COVID-19 and groceries is being around other people in the grocery store while you are shopping.
Should I keep my groceries in the garage or on the porch for 3 days?
This is patently ridiculous. Are you really going to keep your milk, your ice cream, your deli meats outside for three days? This also has very important food safety implications. This sounds like a recipe for disaster, or at the very least spoiled food.
There is a tiny nugget of truth in this advice, because we know that the virus is slowly inactivated at room temperature, with a half-life of about eight hours. But this advice presumes that all groceries are contaminated, and that simply touching the groceries will make you sick, neither of which are true.
Do I really need to disinfect all of the individual boxes & baggies everything came in?
I think that this is also advice that does not make scientific sense. If you are concerned about the outside of food packages being contaminated, I suggest that you wash your hands and/or sanitize your hands before you sit down to eat any food that you might've taken out of those containers. And guess what, washing your hands before you eat is a best practice even when we're not in a pandemic!
Washing fresh produce with soap? Soap should absolutely not be used to wash food. Soap is not designed for food. As mentioned in the linked thread, soap can cause nausea, vomiting and diarrhoea if ingested.
Current recommendations by scientific experts, including the FDA, say to wash fresh fruits and vegetables in cold water. See my earlier answer for more details.
Are reusable bags risky?
Many people use reusable bags as a responsible choice. We do this in my family as well. It's a best practice (even before the times of pandemic) to wash your reusable bags on a regular basis.
While it is theoretically possible that a reusable bag may pick up germs, including coronavirus while in the grocery store, the biggest threat that anyone faces is someone else in the store who has COVID-19. I would suggest that you keep your grocery bags in the car, so you have them handy the next time you go shopping.
If you're concerned that your bags might have coronavirus on them, you can wash them. You should also wash your hands after you have finished putting all your groceries away. This was also good advice even before pandemic.
What I can do to reduce risk when grocery shopping?
Many grocery stores are offering hand sanitizers at the entrance, and are offering to sanitize grocery carts. Both great ideas, and customers should take advantage if available.
My other advice is to make a list, and know what you want, and move quickly and efficiently through the store, picking out the items on your list.
Practice appropriate social distancing, trying your best to keep 6 feet (1.8 metres) away from other shoppers. If there is hand sanitizer available, I also use it when I'm exiting the store, and then I'll use it again at home once I've finished putting all my groceries away and returning my reusable shopping bags to the car.
Donald Schaffner is a microbiologist and expert on food safety from Rutgers University.