Planning meals for a week ahead and having a shopping list keeps you from buying a lot of stuff on impulse which will not be used. On the other hand, you have to be flexible enough to take advantage of whatever is the best value on the day.
Sometimes there are incredible bargains in the "short-dated, must be used today" shelves. Late on New Years Eve I found that Tesco had stacks of things that they wanted rid of before they closed for the holiday - eight salmon steaks for 99p, originally over £12 for example, individually packed in plastic so they froze well and could be used as needed.