For the purpose of passing the visual inspection you'll probably need the muffler. It doesn't have to really do much, just needs to be there. They aren't testing sound emissions and they probably won't ever. Actually, if they ever do, 6.0's are going to have a hard time passing total vehicle noise. Anyway, if you have a muffler, you probably ought to run it for the test if it's an easy install.
Something else to keep in mind, they are only testing opacity with a snap test, not chemical makeup of the exhaust, so as long as it doesn't huff a cloud when you snap the throttle, it doesn't matter if the cat is working or not, you'll still pass. I have a tow tune that would pass with flying colors, and it makes about 120ft/lbs more torque than stock. Just realize, No Smoke = No Problem!
The Current Truck (Oct-2013 to Present):
1999 (Early) F-350 Lariat, CCLB, SRW, Black/Tan
7.3L Powerstroke, 4R100, 4x4, 3.73, 37x13.50-18s, '05 Axle Swap, 6" Fabtech lift, GTP38 "D66" turbo upgrade, SCT 4-position chip with custom tuning by me, Straight Piped 4" down pipe through 5" dual stacks, N-Fab full length triple steps, Pro-Tech cab guard and tool box, louvered 5th wheel gate, Hella 500 Fog and Driving lamps, much more to come...
The one I'll miss, (Mar-2009 to Jan-2014):
2000 Excursion XLT 4x4
6.8L V-10, 4R100, 4.30, 285/75-18, 271whp / 388wtq
The one I won't, (Jul-2005 to Sep-2012):
2005 F-350 XLT, CCLB, SRW, 4x4 FX4
6.0L PSD, ZF-6S650, 3.73, 35x12.5-18, 4.5" Lift