The last thing you need to do is micromanage each other's expenses, a sure road to disaster. But keeping each other informed of major expenditures easily eliminates costly problems like bounced checks or over-the-limit credit card fees.
It's up to you do define major—it largely depends on how much disposable income you have—but many couples use $100, $300 or $500 as the threshold. In other words, if you want to buy a coffee, go ahead. If you want to buy a new television, it's probably a good idea to give your spouse a call first.