The first approach is to picture the entire overview of the cake - it's height, size and shape of each tiered cake and which tiered cakes are stacked and which are not.

Next, determine where the "heavier" designs will be and the "flow" of each design per tier as well as the where the "lighter" designs will be in order to create balance.

Let us example the cake, above.  Notice that the "heavier" design is on the bottom tier.  To create balance, a design using the theme of the bottom tier has been created using a "smaller" version.  (The theme is lilacs.)  The second and fourth tiers have a lighter, more delicate design and keeps the flow going around the side of the cake which is similar to the flow of the bottom tier.  The top tier takes the "upward flow" that also appears in the bottom tier and gives an "uplift" to the cake.  The third tier, which is the middle tier, continues to bring the uplift to the cake and balance the " upward" and "around the cake" flows to the cake.  The hyacinth has been chosen as the flower to create this effect.  (Notice how the decorated cake knife on the left and the bride's "throw-away" bouquet balance the overall theme and flow to the cake.  There are ribbons hanging down from the topper and in the middle tier to "match" the bows and ribbons in the knife and bridal bouquet decorations.)

An approach for decorating each tier would be to stare at the tier and decide where the main color and design should be and "flow".  Then, pipe your design right in the front, center of the cake's side.  To determine where to place the additional, similar decorations, decide how many will be needed to cover the cake's side with even spaces in-between each similar design.  (Notice the hyacinths on the middle tier.  One hyacinth is in-between the two posts and the two hyacinths on each side of the "middle hyacinth" are at equal points of the bottom of the posts which are on the cake tier plate.)

To get a better "feel" for the placement of the designs on each tier, notice that the lilacs on the top tier has formed a design that has been placed on the side of the cake using the posts which are in-between this top tier and the second tier as the guide for equal placing.  Now, look at the second tier from the bottom of the wedding cake and notice the placement of the two different designs - the one in the middle, center front of the cake and the two on each side of the cake.  (Note:  Although the designs are being seen from a given point which is the "station of the viewer", the designs still go all around the cake.)

It is difficult to see the design of the lily-of-the-valley.  However, the design is shown in one of the sketches, below.
 
 

Something to Remember:

"Eye follows color".  Therefore, the colors of any design that completes a creation such as a wedding cake should be placed so that the "eye" can comfortably "flow" throughout the design and overall creation as it "views" the artistic piece of work.

Notice the shades of purple in the ribbons and bows and their placement, the shades of purples in the decorations on the cake knife and bridal bouquet and their placement in relation to the cake as well as in the decorations of the cake.  Notice how the touch of the purple color in the ribbon on the cake topper completes the balance of color which "tops off the cake" and how the eye can flow down the tiers of the cake and to the left of the cake where the decorated cake knife has been placed as well as to the right of the cake where the bridal bouquet is located and then up again to the top of the bow that is on the cake topper.