P235/50 ZR18 tires on 2012 Mustang; door post says 32psi, dealer installs 35psi. Any problems?

Should be nor problem either way. I would usually go with what the door post says because that is what the manufacturer recommends. I have no idea why the dealer would put them at 35.
It's not a big deal though, I don't think you are going to notice much of a difference.

MY MANY REPAIR MANUALS SHOW PICS AND EXPLANATIONS OF TIRE WEAR

https://www.2carpros.com/questions/2007-ford-taurus-tune-up

WHAT OVER INFLATION DOES AS WELL AS UNDER INFLATION

THIS MAY ALSO LET YOU KNOW WHAT TO BE ON THE LOOKOUT FOR

http://www.youtube.com/results?search_query=TIRE+WEAR&oq=TIRE+WEAR&aq=f&aqi=g4&aql=&gs_l=youtube-reduced.3..0l4.1499.4046.0.4589.9.8.0.1.1.0.231.1483.1j2j5.8.0...0.0.DXcQiO37tcc

YOU SHOULD ALWAYS BE ATTENTIVE TO WHAT'S HAPPENING WITH YOUR VEHICLE.....I'M SURE YOUR OWNER'S MANUAL SAYS TO CHECK THE OIL WEEKLY (NEVER KNOW WHAT COULD HAPPEN IN A WEEK, MUCH LESS DAILY)

I TOO WOULD TRY TO ADHERE TO THE MANUFACTURER'S RECOMMENDATIONS

I'M SURE THOROUGH TESTING WENT INTO THE VEHICLES WEIGHT AND LBS PER SQUARE INCH ON THE TREADS.....THE OPTIMAL GOAL IS TO HAVE EVEN PRESSURE ALL THE WAY ACROSS THE CROSS SECTION OF THE TREAD WHERE IT TOUCHES THE ROAD'S SURFACE...THIS LET'S THE TIRE WEAR EVENLY AND GIVES THE TIRE GOOD TRACTION TO THE ROAD

YOUR SAME TIRE ON A HEAVIER VEHICLE COULD REQUIRE MORE AIR PRESSURE--JUST THE REVERSE FOR A HEAVIER CAR, IF FOR SOME REASON YOU CHANGED TIRE SIZE...PRESSURE MIGHT NEED TO VARY TO KEEP WEAR EVEN

I CHANGED FROM 600 X 16s TO 700 X 16s ON WILLY (MY '46 ARMY JEEP)

REASON, THE WW II "NDTs"AND THE LATER "NDT" TIRES LOOK THE SAME, THE 700s ARE TALLER, MORE ABUNDANT, CHEAPER, AND SINCE WILLY HAS 5.38:1 GEAR RATIOS IN HIS AXLES, TALLER TIRES LET ME CRUISE ON THE ROAD FASTER AND MY ENGINE IS NOT SCREAMING AS WITH THE 600s.

OF COURSE THIS CUTS DOWN ON MY TRACTOR-LIKE PULLING ABILITY A LITTLE. BUT EVEN WITH THE "TIRE-MODIFIED-RATIO", WILLY STILL HAS ENORMOUS PULLING ABILITY IN 4X4, AND TREMENDOUS ABILITY IN LOW RANGE.....HIS LIGHT WEIGHT-NESS IS THE DOWNSIDE, BUT YOU CAN ALWAYS WEIGHT HIM DOWN!

I PAINTED ABOUT AN 1/8 OF THE ENTIRE FOOT PRINT WHITE ON A FRONT AND A REAR TIRE. PAINT WEARS FASTER THAN THE RUBBER, SO THIS ACCELERATES THE TESTING, AND IS EZ TO SEE

I BEGAN EXPERIMENTING WITH PRESSURE AND OVER SEVERAL DAYS OF PAINTING THE TREAD AND DRIVING AS MUCH AS I COULD ON REALLY NICE PAVEMENT---AND TRYING TO STAY ON STRAIGHT STRETCHES, I FINALLY FOUND THE BEST(DIFFERENT)PRESSURES FOR BOTH THE FRONT AND THE REAR WHEELS. HE'S LIGHTER IN THE REAR. THAT IS WHERE THE PAINT FINALLY EVENLY WORE OFF OF THE TREADS

THIS IS MY TREADS, NORMALLY PRESSURE IS REDUCED FOR OFF ROAD TRACTION, SINCE I'M MAINLY ON THE PAVEMENT WITH WILLY, I MUST GET THE MOST OUT OF MY TIRES

YOUR TURN

THE MEDIC