The screw is just to help locate the valve stem and prevent it disappearing into the wheel when you try to attach the pump. It is not necessary for it to be tight, although if loose it might rattle in an annoying way. The valve-cap is provided just to prevent the tube being punctured during shipping when it's all folded up in its box, but I tend to put them on as dust-caps, and to prevent the screw falling off when it works loose. HTH -

- Wil