That's a great question! And the answer is...both. It completely depends on how you approach your month. I used to shop the flyers- every Thursday night when the new flyers came, I'd find out what was on sale, and plan my meals and grocery list for the following week accordingly.
Over time, though, we've started to do more of our shopping at one store, rather than several. We found that for about the same price for most foods, the quality of just about everything is better at Costco. Because of this, I now meal plan for the month, rather than the week, and do one big grocery shop based on the entire month's plan.
For the remainder of the month I shop locally to pick up fresh fruit, vegetables, dairy, etc. as needed. I'm looking forward to checking out the farmers markets this year!
How do you shop?