Here are some tips related to the creative tab (i.e. the GUI where you can, if in creative mode, select all the blocks and items to build with).
Controlling The Order Of Display In The Creative Tab
The items and blocks in the creative tab will appear in order of ID, which may be roughly in order of registration but generally may not be controlled. Furthermore, if you want your custom blocks and items to appear somewhere within the vanilla blocks and items then you will need to control the display order.
Check out diesieben07's Tutorial On Controlling Creative Tab Order.