New MTB tyre looks wobbly I recently installed a pair of new tyres to my MTB. The front seems fine however the rear looks like it is not spinning completely centrally.

This is a rather peculiar situation because the wheel seems centered but the tyre is off center in some places.

I do not feel that anything is unusual while riding in that there are no vibrations or wobbles.

How do I correct this visual anomaly, and is it imperative that I do?

These are 26" MTB tyres with tubes.
Let all the air out and work your hands around the tire, "breaking" the tire edge from the rim where it has stuck.  Go around twice doing this.  Then reinflate slowly, checking every few pumps to see if the tire is properly centered.  If not, work it back to the center with your hands.  

Pay particular attention to the part of the tire right next to the rim -- there's generally a sort of ridge in the tire sidewall that, if the tire is properly seated, will be a constant 2mm or so from the rim edge.

If you cannot get the tire to seat, regardless, then you may have the misfortune of having mixed fractional and decimal tire sizes.  But first make sure the tube is not twisted inside the tire.