Responsa for Avodah Zarah 110:3

They responded: it is a well-known halacha that something is only considered 'wine' once the liquid begins to flow to the mouth of the winepress (Avoda Zara 55b:3). But the squeezing of grapes inside a vessel is no different to packing a sack of wheat in order that it hold as much wheat as possible. So too, the squeezing of the grapes here is only to pack them in tightly, and there is no hint of transgression. Somebody who purposefully refrains from doing something permitted is considered vulgar. But as for you, "Do not disdain your mother of old," (Proverbs 23:22), and those who are stringent in this regard will be blessed, while those who are lenient, [Heaven is] lenient with the length of their days and years. All this they responded to me.^*The Talmud (Avodah Zarah 75a:6) seems to say that vessels that carried grapes need to be rinsed, and while this line is omitted by the Rif, Rabbeinu Tam (cited in Beit Yosef Yoreh Deah 138) requires it too. Perhaps this is the source for the discrepancy of customs being discussed here. As usual, the heavenly response gives great importance to local custom.