The best policy is one of enthusiastic mutual agreement. That means no one is having their arm twisted or having to be sold on what the other one wants. Even if your wife says, "Just buy whatever car you want and I'll be happy", don't accept that. Find some that you like. She won't enjoy driving as many cars as you will, but take the short list to the house, give her a ride and watch her reaction. Buy the one she likes best. For example, you would be happy with a Buick LaCrosse, but that used Buick Lucerne is only $1,000 more and it is nicer. If she really wants you to have the Lucerne, buy it. But if she brings up the slightest damper on spending the extra money, let yourself be influenced by her, and let her know you appreciate her common sense and frugality.
Do that on the big things, and when you have a bunch of cash laying around and want to spend $1,000 on a Zeiss scope, you won't have to mention the $1,000 that "you" saved on the car. She'll do it.
I buy, sell and trade guns. My money for it comes out of my own trading. Any recreation comes out of extra work I do, not out of my regular, expected income. If I am billing by the hour, it comes out of overtime. And it is only a fraction of that - she gets the bulk of it to save for the new things she wants to improve the house.
