Never shut off a valve in front of the pump with the pump running. This will cause the pump to catch air or cavitate, which causes problems. If you want to reduce the flow going to your sand filter, put the valve between the pump and filter. This allows you to shut the valve and slow the water going into the filter. This can also be a bad idea if you end up making your pump work outside its efficiency curve. It would be difficult (read: nearly impossible) for you to tell when you are in and when you are out of the proper curve. I wouldn't worry too much about that though, just don't shut the valve much past half way.

Brad
www.waterworkspools.com