Hi, yeah I have a set of the 17" YR1 flakes. First off, you won't be able stagger them (17 front, 18 rear) because the 17s and 18s have different rim lips (17s have stepped lip like in my avatar, 18s have rolled lip like on the Burt Reynolds Edition T/As).
As to backspacking, it sorta depends on what you're going to be doing with the car. I ordered their 17x9s all around with 5" backspace (which would be 0 offset), but it turns out that they are advertising them incorrectly as they are really 17x9.5 inches.
If they'd have told me that I'd have probably gone with 5.25" all around... or gone with 17x8s up front with 4.5" backspace and 17x9.5" out back with either 5" or 5.25". Here's a pic of a set of 17x9.5s all around with 5" backspace. It looks great, but the owner of this car has an Air Ride suspension that he uses to lower the car way down and therefore the 5" backspace was not quite enough for him... especially up front. (BTW, tire sizes on these pics are 275/40/17 up front and 285/40/17 out back).
Either way they are awesome rims and I can't wait to see them on my car. Here's a short vid of my rims that were custom painted for my 77 Y81 SE.
