While not a huge distance I would say that Monmartre is NOT that central.
It is inside their orbirtal motorway, but still some way out of the centre. According to Google maps it is 4.2km (2.6) miles from Sacre Cour to Notre Dame which is roughly the centre of Paris. It is 3.6 miles to the Eiffel Tower.
Years ago it was a separate village before Paris grew and encircled it.
I am not saying it is a problem being out there, but you will have to get a train (or walk) to get into the centre of Paris and to the main attractions.
There is a map here showing where it is (see on the right)
http://en.wikipedia.org/wiki/Montmartre