I think square is better,Especially on a lowered car hard to tell that the front tires are smaller unless it’s extreme.
I guess if you can’t get wide enough tires on the front to make you happy for what you’re trying to do then going with the widest possible in front and then slightly larger in the back can be a good option. I like having wide tires on the front also for the best cornering possible. But for the most part in my opinion go as wide as you can in the front and then match it to the back. And then if you need more or the back could always get a set of drag slicks just for the back.
I personally think the S197's actually look better squared up and lowered. Staggered looks better on the SN95/New Edge Mustangs. Of course, it's really personal preference. Post'em up when you get your Bullitt wheels on your Mustang.