I think that's no longer true for modern flatscreen TVs. Older CRT TVs had caps charged to a dangerously high voltage, but all the caps on an LED/LCD TV should be a low enough voltage to be touch safe.
Mains will still kill you, even if its not 3.3kv or what was needed to run a CRT.
Its always best to work out where the mains PSU is, and either cover/isolate it so that you don't touch it by accident. Even if turned off, there is a non-insigificant risk that there is either mains power lying around or some other large charge/current ready to bite.
THe back of the inlet is also tends to be a hazard, so be really careful around there too.
Can't echo this enough. If you're working on anything that has been connected to residential mains... use a multimeter. They're cheap! Check the power supply for residual voltage from PSU caps. Discharge them safely before working on the equipment!
Some older flat panels use “cold-cathode fluorescent display backlights” and they’re driven by a high-voltage inverter, I believe - so it’s still a good idea to be wary.
Except for the switching power supply, where you have capacitors charged at (rectified) mains voltage so around 155 or 325 volts depending on where you are. They should discharge relatively quickly, but they might be dangerous for a few minutes.
