This is a small tip, but one I used to suffer from a long time ago…Who knew the solution was so simple?
When using textures with alpha channels Maya can get confused about which areas are actually transparent when rendered. Below is a single plane textured with an alpha channel, as you can see the whole plane is casting a shadow onto the box, rather than just logo.
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• To solve this simply open the Connection Editor by going to Window > General Editors > Connection Editor.
• In the left hand side select and add the file node which points to the texture.
• In the right select and add the actual shader node.
• Now connect the outAlpha attribute from the file node to the shadowAttenuation attribute on the shader.
• Now when you render the only shadow you will see will be cast from the opaque areas of the texture page.
There you go, told you it was simple 
