When in a power off configuration, the aircraft will glide best at the Vmd (Minimum drag speed). This is the point at which lift dependant and form drag meet on a typical drag curve. Therefore, if flap is lowered the form drag will increase, as will total drag. This is not the best configyration and a clean aircraft flown at the correct speed will give the best glide range. By lowering flap, you could achieve a higher glide angle to clear obstacles, but this would lead to lower range.