I always recommend reading the earlier books before the later ones bc that’s the order in which AR explored the characters. #READ THE BOOK.gif
The crossover books will spoil things or may leave some things confusing/unexplained which happened in the original Mayfair series, so if you don’t want to be spoiled that way, y’know, start at the original books.

[art by Thomas Blackshear, very Gustav Klimt, oui? A good Merrick.]
I’m not as familiar with the Mayfair series* bc I read them a long time ago and didn’t reread them… but you’ll probably see the witches as richer characters when you know what they’ve been through before the hybrid novels (AR calls the crossover books “hybrid novels”). IIRC, sometimes past events are referenced in the hybrid novels, but not fully revisited from other POVs as happens in VC.
*One exception is Merrick Mayfair, who isn’t mentioned in the original Mayfair trilogy, she’s in Merrick.
For example, Mona Mayfair is no ordinary witch, and so when we see her in Blood Canticle (Mona was so awesomely sassy in BC!), it feels like she gains back an inner strength about herself (from the earlier Mayfair books) that may have been lost.
Also relevant: at the booksigning last night (11/30/16), someone asked AR if we’d see more of Rowan or the other Mayfairs in future books, and AR basically said she doesn’t think so, bc the hybrid novels haven’t worn well over time, and that she feels the original Mayfair trilogy is good as it is (also, she wrote it in a house in NOLA she no longer owns; it’s harder to get back into that universe from another location). We do have some Mayfair mention in PLROA tho, so clearly AR is keeping some things from the hybrid novels and tossing others so i d e k.