Hi,
Most 2d ui frameworks will allow you to anchor to screen left, top, top left, center etc etc, I strongly suggest you do that, 2dToolkit and nGui have that for sure, I use it all the time.
Now, if you are only using guitexture, you can do so to, by changing the transform position, they range from 0 to 1, you can achieve anchoring like that too.
With Unity GUI, you can do so too by using guiLayout stuff.
bye,
Jean