Of course, last year I followed up my Zola purchases (Les Rougon-Macquart and the other stories that don't follow the cycle, making up the 6 Pléiade volumes) with all of the Balzac fiction (La Comédie Humaine in 12 volumes, and two volumes of "Œuvres Diverses" from Pléiade). I haven't gotten around to the reading but I always assumed I would read Balzac in whatever order I felt like but try to read Zola following the series order.
There are dozens of ways to tackle Balzac, the one he established himself for the Furne edition (and that Pléiade largely adopted, IRRC) is just one of them. Most people I've known go on whims based on what they feel to tackle next, the works are all independent and Balzac merely regrouped them according to themes or what he felt was a logical progression (it's all a bit didactic). Reputedly (so we're told by lit. teachers) the reading order influences the perception by the reader of his vision or some themes, but I can't vouch for that.
Any Zola novel can be read on its own - the ties between them are usually not that elaborate and the stories are pretty self-enclosed, but personally I think it's more interesting to follow the reading order Zola established, if only because this way it's much easier to follow the evolution of the social and political background and the family history unfolding also makes more sense this way. There's a also the matter of recurring characters. Zola doesn't waste pages rehashing what happened to them previously. It's not required to follow the plot (when it is, he'll mention what is relevant), but it's far more interesting to read about, for e.g., the crook financier Aristide in the second novel when you know how his personality formed back in Provence in his youth and then with his parents during the coup/revolution, which was told in the first novel. Otherwise, he might appear not that well developed a character. In the second novel you're just told his parents have risen to prominence back in their small town following the revolution and that he's jealous of his elder brother, now in the Emperor's cabinet and who keeps him at arm's length. The first novel explains all the details. It might be very secondary if this was your run of the mill family saga (or when it's read for the plot), but with Zola it's far more social studies than anything, so those details matter a lot to the global perspective and the portraits he was trying to make, I'd say.
