I'll take photos of wineries year round... who wouldn't... the photos pretty much take themselves. That said vintage is the best time of year. Pickers in the vines, some leaves starting to turn, winery in overdrive crushing, macerating, punching-down, barreling... so many opportunities for action.
As the lead photographer for the annual 'Wineries of Victoria' magazine (Australia) for 5 years I have seen many types of winery operations from massive factories where volume is key to family hobby farms that produce really personal and unique wines in barrels you can count on one hand and everything in-between.
I like to shoot wineries in a documentary style, I think it lends well to a feeling that the wine is coming from a genuine, real place. That said I can shoot in a more polished commercial style too especially if you have that style of cellar-door and hospitality operation.