As a general rule of thumb, there should be one or two good sniper spots, usually covering the quickest, most dangerous path. Make sure there is at least one good path that circumvents the sightlines though, as options like that are what make good maps fun. These side paths need to be designed such that they are not sniper friendly (many twists and turns, no long sightlines that snipers could use).
Also, as grazr said. In areas where there aren't clearly defined paths, but instead are just large battlegrounds, adding in obstructions such as huts, rocks, or little walls go a long way to reduce snipers' effectiveness (Gravelpit point A really is a perfect reference for this).