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Keeping your advisor up to date with how your work is going is important. You might think that your work is going along the correct path, but it is quite possible in research to start heading off down a path which - while possibly interesting - may not be the route your supervisor wants you to go down (at least, not without discussing it first).
One way of damaging a good working student-advisor relationship is to end up in a situation where your advisor is demanding to know why you have taken the research in a new direction, without consulting your advisor for his/her advice.
I do not think that a monthly update is going to overload your advisor's email account - unless he or she is an internet hermit. If you are worried about annoying your advisor with information overload, you could always end your initial email reports with words indicating that you are more than happy to talk face-to-face about your progress, or say that you are happy to make the email updates less frequent.
No advisor wants to be left in the dark about what their students are doing. A good advisor will not consider a regular email update from their student a negative prospect - so long as you don't try your advisor's patience with multi-page long emails. For an MSc thesis project - where time is tight - I think that reporting on a monthly basis is about right.