The big question is "What type/weight aircraft". That will determine how low you can run pressure. If you're putting them on a Cessna 180/185 or similar I'd keep pressures at or above 20 psi. A cub could run down to ~ 12 psi.
Go to NAPA and buy a digital air pressure gauge, remove those hub caps so it's quick and easy to check pressures, then CHECK them frequently. Unless you're using nitrogen, all tires/tubes leak some. Temperature changes pressure a little, etc. so, if you're going to run low psi, make checking pressure part of your routine.
The cement thing does help.
Finally, the Goodyear 26 inch tires are great tires, but they are NOT very flexible. You can run the pressure down to scary low, and they still won't flex much. If you need a tire that will roll over big stuff with a lot of flex, look at Bushwheels. Just don't expect the GY 26 to perform like Bushwheels just because you lower the psi.
FWIW
MTV