JBert and 5.0 Candy have offered some good advice when selecting and why. Fortunately out here in Cali where I'm at the weather doesn't get to drastic one way or the other. But in addition to the suggestions mentioned by the above mentioned Google is a great tool as well to get a look at what they might look like on the wheel as well.
On my 2012 GT I'm running Shelby CS69 Magnum 500 18"x9.5" with BF Goodrich g-force Sport Comp-2 275/40/18 all the way around. Although I do like the staggered look as well. Obviously some drawbacks to going staggered.
2012 Mustang GT Grabber Blue 6spd (Current)
1991 Mustang GT Ht Oxford White (Current)
1991 Mustang GT Conv Black
2000 Mustang GT HT Silver
2007 Mustang GT HT Tungsten Gray
2010 Mustang GT HT Black
2010 Challenger R/T Torred(Current)