I think the key is courtesy. Card everyone, but be extremely polite about it.
I owned a liquor store. If you didn't have ID, I wouldn't sell to you. Even if you looked 45.
I was the ONLY business in my town that did not have employees arrested during a sting operation. It's a tough way to learn a lesson -- being hauled out in handcuffs, and being fired, then facing court and fines. The business is also in danger of losing its liquor license.
That's why we shop. Much, much better for a company to pay a shopper fee and learn that the employee is NOT carding; the employee can be retrained.