Four small western Nevada County wineries hope to increase their visibility and sales by opening tasting rooms in downtown Grass Valley.Sierra Starr Vineyard and Winery is convinced it's a good idea; the Grass Valley winery has seen its sales increase more than 50 percent since opening its tasting room last July at 209 W. Main St.Not incidentally, the proliferation of tasting rooms along West Main and Mill streets gives visitors another reason to spend time in the historic 13-block district.Tasting rooms have been a familiar part of the downtown Nevada City scene for several years, but their arrival in Grass …