Borders can affect the way you view the image. To determine if you need a border you need to ask yourself where you want the viewer's eye to go.
My personal favorite style of pattern making is to give your pattern a photographic look. Your subject matter can act as a border. Use objects like trees, peepholes or columns to make a natural border. Another way is to make the subject itself become the focal point by zooming in on it until it’s the only thing you see. This technique will draw your eye naturally to your focal point making a glass border unnecessary.
A reason to add a stained glass border is to create a matting effect. It, therefore, gives your eye an edge to pull it back to the center focal point. This technique is especially helpful when you have a subject that may need to have too many background break lines (like feathers from a bird or petals of a flower). In this case, you can cut the background down to practically nothing and add a border so it doesn’t feel so messy. Thus bringing your focus back to your subject and not all the messy cut lines.
To sum up, adding a border or not is more of an emotional choice. What kind of feeling do you want to convey?