A: Really bad.
Grandma may have done it, but that doesn't make it ok. Even if you rinse it out before reusing it, the bag can still contain bacteria from food and your hands.
"Everytime you reach in, you leave behind germs," says Joan Saige Blake, R.D., spokesperson for the Academy of Nutrition and Dietetics. "And once inside the bag, those germs can multiply to levels that could make you sick."
Putting baggies in the dishwasher won't cut it either, since this kind of plastic isn't made to withstand high heat.