Which tire combination?
First part...Don't listen to ANYONE who thinks staggered width wheels definitively hurt handling. If it did, cars like the Dodge Viper, Porsche 911 GT2/GT2 RS/GT3/GT3 RS, Chevrolet Corvette, Lotus Elise/Exige wouldn't have staggered tires from the factory (and FYI, the Viper ACR is the best handling car in the world and has staggered tires).
Now, speaking from experience...I had staggered tires (not wheels, just tires) on my 98 Cobra. I had 17x8s all around with 245/45-17 front and 275/40-17 rear tires. The car handled better than it does now with all 245/45s, and I had no issues with needing to rotate the tires. The tire shops can swap them left to right and inside to outside for even wear, you just can't rotate them front to back, which is no big deal, really.
Now, you didn't say what model mustang you have, but I'm guessing V6, otherwise you would have mentioned it was a GT. Just a heads up, if that's the case, stick with 18x9 all around and just use slightly wider rear tires (not drastically wider like mine were), because the V6 has a hard time spinning even a 225/45-18, so a staggered 225/45-18 front with a 245/40-18 rear would work fine if you go that route, but I'd get 18x8 wheels instead of 18x9. The bigger wheels and tires will slow the car down and hurt the ride quality more than you could imagine, going from a 16" wheel to an 18".