Perhaps it has been described as "hit and miss" to just pop in at wineries, but I shouldn't say that it's frowned upon - on the contrary, in a lot of places you will be warmly welcomed (as long as you avoid the lunch hours (so from 12.30 to 15).
South of Pisa you have the Colline Pisane region with some very good wineries. To the North, in the Colli dei Luni, I have actually never been so I am not so sure about routes or wineries. But here are some web-sites: www.biologicoboriassi.it / www.villapavesi.it / www.poderelavandaro.it
You might want to check out the location before you go, as they probably won't be on your main road.
Best of luck!
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