It is indeed well worth my while to check the newspaper and sales flyers for lower prices. If I can save 25% on tomatoes, it is money in my pocket. Especially on items that do not spoil, like sugar, paper towels, dish soap, motor oil, olive oil and such, you can save a whole lot by buying it on sale and stocking up. You can save even more by shopping at the cheapest stores that are on your way home from work or from some other place you were traveling to.
Costco, BJ's, Sam's Club (I have belonged to them all at one time or another) are only really useful if you have a fairly large family. If I buy two gallons of milk or soymilk, one will spoil before I can use it. Costco would be fine if they would sell imperishables of regular quality for a low, low price, but they don't normally do this: they sell huge sizes of premium quality stuff, and you don't actually save at all.
I gave up on Costco when they had no men's white t-shirts at all. None, only tshirts which cost $6.99 and advertised the Pacific Ocean. I realize that the same t shirts would have cost $10.99 in the Mall, but it was my feeling that I do not really want to use my bod to advertise any ocean.