Personally I use tile usage before I start templating any terrain. That means that I place squares that touch each others' corners to create a large grid over the map that is centered to the center of the map (the ground then reminds a chess board).
However if you have already created other half or a quarter of the map already and it is not centered perfectly but is otherwise like you want it to be, the best way to fix any mirroring / symmetry issues is by mirroring the terrain. You can do this by painting over the desired terrain (select it by simply holding down left mousebutton and drag along - the terrain will be selected). Once you have selected the desired area just press Control + C -keys to copy the terrain. Now you can paste the terrain by pressing Control + V -keys and you are able to place it down. You can either shift the terrain towards a direction you wish to shift if towards. While you have the wanted terrain pasted, but not placed (after you've pressed Control + V) you are also able to rotate the terrain and mirror it around itself by going to the Edit menu in the top-left bar of the editor. There are options to Mirror or Rotate the object currently being placed (the one you created using Control + V).
coll smilies btw 
