Not all the things you’ll ever roto will be crisp. Come to think about it, there is very little roto work in which at least a section of the focus object doesn’t have some sort of edge blur.
Although you’ll want your spline to have a blur that matches the focus object, you don’t want to overdo it. When considering a focus object’s blur, you want to nail down the kind of blur it is. If that object has a consistent edge blur, like a fuzzy sweater, or an element in the distance is blurred by the camera, you’ll want to find a blur setting that matches that particular focus object and keep it at that amount for the duration.
A good rule of thumb is anything less than a 5 to 10 pixel blur. Whatever the mattes will eventually be used for, you ...