Trimming the alpha isn't the issue. The real issue for me is having 50+ UI pieces that need precisely placed in NGUI, exactly as they are placed in a photoshop file. This means I want to avoid anything that requires I write down the coordinates of each piece, add the center of the sprite width, and type that in NGUI for each sprite.
Older NGUI to work like this: export small sprite as a fullscreen image. Import that image into Unity and drop it into an NGUI atlas. NGUI atlas was smart enough to trim the alpha, use that info to offset the sprite for you (!!!!!) and make the bounds of the sprite as tight as possible! Done! want to reposition the image in photoshop? go ahead! export image, update in the atlas, and whammo...the new position is calculated. All my previous projects used that workflow. Now I updated NGUI and whoa, it does NOT do that anymore. Now I have to read the padding info, write it down, zero it out, make the widget, add an anchor, set the anchor gameobject container to a fullscreen object, add half the sprite width to the padding, add half the sprite height to the padding, set the anchor reference, type in the x pixel offset, and the y pixel offset. For 50+ sprites.
Now you see why I am miffed. It used to do this. FastGUI doesn't support the latest Unity/NGUI, and it costs money when NGUI used to do this :/