TPMS Is a good system but never perfect. My first experience with TPMS was in a rental 2009 Chevy Impala in Orlando, Florida. (Enterprise), in January 2009. The following day after I rented the Impala, the left rear tire gave a warniing on the DIC (26 PSI), while the other three were at 31 PSI. I have no air pump, so I visited a local Walmart and bought a cheap 12VDC air pump and a mechanical tire gauge. When I measured that tire, its pressure was 29 PSI, so I used to pump to increase its PSI to 31. The gauge measured 31 PSI in all four tires. But the DIC still had the 26 PSI reading.
When I returned the Impala after the week, I reported the problem to Enterprise because the next rentee will probably have the same issue. Now I have a 2009 Chevy Equinox, and the TPMS reports all tires one or two units below the mechanical gauge PSI readings. So I don't trust the DIC readings 100%. I prefer to go "old school" (as posted above) and manually measure the tire pressures, but I do use the Equinox DIC as a warning when the TPMS PSI reading drop below 30 PSI.
I have also seen and read about the Ford system. Their TPMS PSI readings are taken by the BCM but stored, and the DIC only reports the "low pressure" warning when the TPMS reading drops below a specific level. It doesn't report the actual TPMS PSI reading, nor give you which tire! A Ford owner must carry the gauge to find which tire has the low pressure (The new Ford cars do come with air pumps in the trunk, just as new GM cars do).
So the GM TPMS is more user friendly, but I will never trust their readings completely.