Shirts for use without ties under sports jackets

Here's something that I've wondered about for some time. What is considered the appropriate kind of shirt to wear in casual situations (without a tie) under a nice sports jacket? Is a dress shirt, that is usually worn with a tie and having a stiff collar, appropriate? Or should we not use a dress shirt in such a situation and instead look for a truly "casual" shirt that would never be worn with a tie (less stiff collar, etc.)? A turtleneck works well under a jacket, but I'm concerned here with button-up shirts.
Roger