Spacing is a vital aspect of a WonderBoard installation, as the best way to cover the joint will depend on how wide you have spaced the joints between the pieces. Tile installations require some elements of elasticity to absorb movement from the shifting of the home; if you install the boards too tight, the floor will buckle rather than absorb the movement during the expansion of the home in the summer months.
Always use at least an eighth of an inch but no greater than a quarter.
Mesh to Mesh
WonderBoard is meant to be installed factory edge to factory edge when possible. While you will always stagger the joints to ensure stability, when you install the boards mesh edge to mesh edge along the perimeters of the pieces, you do not need to add any sort of mesh tape on top of the joint.
This is because the mesh on the edges acts as in the same way as a topically applied layer of tape. The joints still need to be filled but not covered.
Any place where you have a cut piece of WonderBoard up against a factory edge, or a cut piece up against a cut piece, you will have to include a layer of mesh tape on top of the joint to help hold the thinset mortar in place and help with overall flexibility. Drywall mesh tape works great in this instance, as long as the floor is clean.
After the tape has been applied, you can cover the tape with a layer of thinset mortar and finish the prep steps for your tile installation.
The only areas you need to be worried about when you are mudding on WonderBoard are the joints. You generally want to apply the tape and spread the thinset over the tape 24 hours before you do your installation so that the mortar has time to dry.
This allows you to do your layout without fresh thinset all over your tools. The flat side of a trowel is used to mortar the tape into place on the face of the boards along any break points between sheets.
The perimeter is left untaped and unfilled for expansion purposes.