Over or under? It depends on how you want your shades to look, how much light you want to block and other factors:

The most common option is the standard roll, in which the fabric rolls back, close to the window glass. Having it as close to the glass as possible blocks the most amount of light and affords the most privacy,so that’s why a standard roll is recommended for small reveals, meaning the window depth is shallow, because it gets the fabric farther inside the opening than if the roll were reversed. The downside for a standard roll is that the roll and hardware show, a look some people don’t like. If that’s the case, valances can hide it. This offers the most finished look but definitely increases the cost. In some cases adding a valance can double the cost of the shade. Also, valances cover up decorative molding, which may not be appealing for some.

A reverse roll means the fabric comes over the front of the roller. You don’t see the fabric on the tube, so this creates a more finished look without the need for a valance. Reverse rolls are also used for shades that have two different colors on each side of the fabric, if this is the case, for example, with blackout shades that have white on one side and color on the other, hanging the roller standard would mean that the roller would show all white while the part of the shade that hangs down would be black. A valance could conceal this, but again it’s an added cost and might look too fussy in a minimalist setting. The reverse roll is also good for clearing an obstruction, like a window crank or door handle, because it pushes the fabric away from the glass.