Within the first 24-48 hours, I generally always test when I get an -unexpected- low. If it's daytime, and I've been slowly trending downwards and its been 5 hours since I last ate, I trust it to be low because that's to be expected.
At night, I've learned to look at the trend. If it was a sudden, sharp downwards trend that set off an alarm it was probably a pressure-induced low from rolling over onto the sensor. I'll wait for the next reading and see if it trends upwards. If it does, I'll usually ignore it and go back to sleep. I've only had this a couple of times as I rarely sleep in a position to roll onto my sensor. If I've been trending slowly downwards and am being woken by a Low alarm, I'll test to confirm if it's within the first 24-48 hours of the sensor, or if I've restarted the sensor and it's gotten to where when I go to calibrate it's no longer close, which indicates that it's reaching the end of its life. The last couple of sensors, after the first 24-36 hours or so, my Accu-Check and my Dexcom tend to be within 5-10 points from each other as I have a very strict calibration protocol and only start sensors and calibrate at the most optimum times.
Basically, between days 3-12 or so, I trust my Dexcom enough to use it to treat lows and highs, and to dose my insulin off of.