Here's what my favorite WT book says:

Heavy or stiff fabrics such as damask, brocade, and lightweight fabrics that are lined and interlined: 2 to 2 1/2 times the width of the rod.

Medium weight fabrics such as chintz: 2 1/2 to 3 times the width of the rod.

Lightweight fabrics such as gingham, taffeta, lightweight silks: 3 - 4 times the width of the rod.

Very light and soft fabrics such as voile, tissue-weight silks: up to 5 times the width of the rod.

My two cents worth: if you never plan to close them you could get a way with less width.