We are going to Mendoza in October of this year. Do we need to look at booking winery tours now or should we wait to see what is available in the city at different tour options?
Either way. October is still not super busy so I am sure you won't have a problem getting into any wineries...except perhaps for Catena Zapata. Only thing to pay attention to is that on Sundays, most wineries in greater mendoza are CLOSED. And in Valle de UCO, all wineries are open on Sundays but they close on Mondays.
You definitely won't have to book that far in advance, it's still low season. I went to Domiciano and La Rural and just showed up, it was totally fine. I wrote up about my experience here if you're interested in reading: biancarods.com/home-3/a-weekend-in-mendoza