A power tap is NOT able to zero itself in general and can record unrealistically high values if there is a positive offset. It is able to auto zero when a negative torque is measured, but for cases where the offset is positive, the unit can not correct itself. I recommend zeroing the unit before every ride (it takes about 30 seconds) and checking calibration about every six months. One method to check calibration is here: http://www.midweekclub.ca/powerFAQ.htm#Q23

Using a Computrainer to check a powermeter is very risky. As mentioned above, unless recently calibrated by RacerMate, Computrainers are generally less accurate than the powermeters they're being used to check.