The tach may be the first gauge to go, but the temp gauge is also very prone to failure. Eventually they may all go haywire.
The stepper motors are available on eBay for about $7-8 each, and you need six for the entire cluster. It requires your patience and a little skill with a soldering iron, but its not difficult if you just take your time.
I used a different process than in the video above to remove and replace the needles, but it worked out the same. I found the information on removing and replacing the needles in the documentation for a Simco white gauge cluster overlay.
In general, there are no sensors directly connected to the gauge cluster in the 2005-2010 Mustangs. The sensors report readings to the computer and the computer drives the gauges. If you want to verify the actual readings you can use an OBDII scan tool to read everything right off the maintenance port under the dash. This will tell you what is coming from the sensors.