PHILOSOPHICAL & PRAGMATIC RESPONSE
In a previous post you wondered if 17"-front/18"-rear would look 'dumb'.
Within reason, that's a personal call no one can make for you regardless of their opinion. Just like a guy's preference in women, I've seen some atrocious (to me) wheel styles/set-ups that other guys rave on about. But I won't openly judge their choice anymore than I would diss the looks of their girlfriend or wife - I'm not married to her and, if he's happy, all the better for everyone. It's a beauty/beholder thing.
That said, I humbly offer my small opinion.
For a few years now I've also considered this same set-up (17"F/18"R) on my '87 GN. From what I've seen, it's not a common application on most cars, Buick or otherwise; most seem to define 'staggered' as differing widths front-to-rear so the addition of differing heights would give your GN a unique talking point. Consider also the G-body profile: the rear wheels sit under both the trunk and a portion of the rear passenger compartment giving the car a 'thicker' feel in that section whereas the front wheels sit beneath the 'thinner' front end. Given these dimensions, an identically-sized wheel/tire combo (like from the factory) would tend to enlarge the appearance of the front rims while the thicker rear section would tend to minimize those. Increasing the rear diameter would serve to equalize these proportions. Its a subtle observation to be sure and a 1-inch difference at those dimensions is negligible. Depending on wheel widths and choice of rubber, it may not be noticed at a glance. But it would register subconsciously and, most important, you would know. As with the human body, stance is about symmetry. Your average Buick buyer in the '80s wouldn't care about this, only that he could easily rotate his tires all around and the spare would be of the same diameter. GM, too, had expense to think about so identical rolling stock would keep their bottom line in check.
Another VERY practical consideration: availability of rubber. I first researched a staggered look back when 16" & 17" tires were the pinnacle of performance. Not so today. 18"s, 19"s & 20"s are entry level from the factory and, arguably, tire suppliers are only going to carry what they can sell. 16" rubber in a variety of sizes is scarce now which makes me fear for 17" in the coming years. Therefore, I chose to research sites like TIRE RACK where I could plug in a specific number in their calculator and discover how many brands offered that size. The more brands that did, the safer I felt about its availability in the coming years. Based upon the tire's specs, I then chose my wheel widths accordingly. Admittedly, its a stunted way of going about things but I cannot afford buying another set of rollers just because proper shoes are no longer available for them.
What I settled on was 17x8"-front, 18x9.5-rear. However, when I finally do scrape together the cash to buy my wheels, of necessity those sizes may change again. Bottom line: do your research.
For what its worth, I hope the above helps in your endeavor. Be sure to post your final choice when you make the leap - we're all rooting for you, including my young daughter who gives you a thumbs up ; )