Richard,
I use stem extenders for my inside tires (the solid metal kind, not the flexible ones) and thread on the sensors for my TPMS. Works just fine and I don't get any leaks. If I did, the TPMS would let me know.
A bit of additional information. NEVER use the lead bead style tire balancing stuff that they put inside your tires. It can get into your valve stems and cause the valve to stick open when checking tire pressure. The tire goes flat and the lead then gets into the bead area. When you re inflate the tire you get a slow leak. They can't find it in the water tank at the tire store because there is no weight on the tire but put it back on your rig and presto, the leak is back. Only cure is to completely dismount the tires with the lead beads, completely clean them out on the inside and the rims and replace the valve stems after completely cleaning out the stem tube.
Been there, done that.