This is good advice: Why It's Not A Good Idea to Use Up All You Have Before You Buy More
When you are trying to be economical, you don't want to be put in a position where you HAVE to buy something right NOW. Urgency is a great enemy of frugality.
By the way, it's an enemy of time management, too. You want to do things "decently and in season" -- not scrambling at the last second and having to squander your precious energy to get it done.
Sometimes there is simply urgency, and you can't do a whole lot about it. There was one time we had to rush Aidan to the emergency room 60 miles away and from there he had to go straight to San Francisco 250 miles away. I stayed with him, of course, but I had only the clothes I was standing in. My husband had to go and buy me some jeans and a shirt. He didn't have time to scour for deals.
So sometimes, you hit urgent situations that are somewhat unavoidable. After that, though, I kept an overnight bag packed. You want to minimize those crunches as much as possible.