Hi Kanad,
Usually we do a 'pre-test' with all subjects before starting. Many people forget they have dental work. It is very efficient to ask them to blink their eyes a few times, open and close their mouth a few times and take a couple deep breaths while you observe the MEG signal. This will give you a good idea about any artifacts.
With regard to the overall/continuous SSP, yes, just one event at the beginning of the continuous file. Then compute the SSP using about 200s of recording. This should give you a topography of the ongoing artifact.
Here is a sample of the jumps I see:
I found 185 of these using MRT57 for the detection channel.
Beth