Why would starting up the serial monitor function cause servo's hooked up the arduino to jitter?
When you open up the serial monitor it causes the attached arduino to perform a auto-reset, start of the bootloader code, and finally a jump to your sketch. This is most likely the reason you are getting a servo jitter. To test this, instead of opening the serial monitor just press the reset button on the arduino and see if the servo reacts the same.
Are there any pitfalls to watch out for with servo control, where you get strange results of unexplained servo movement from either peripheral devices or code?