I know some simple pieces where the initial phrase starts in the IV chord or the V chord, but I'm wondering if Bach used to do that.
Generally speaking Bach likes to establish the home key chord at the beginning of his pieces, but I did find some exceptions on a cursory look through my scores. For example, in Book II of the keyboard Partitas, the Courante of No. 5 has an upbeat based on the dominant. And the Gigue of the same Partita does something similar, with the home key chord of G only happening on the second beat of the first full bar. I am sure with a far fuller investigation, many more examples could be found.