If you do a google search on 'aquascape' and 'golden ratio', you'll find a number of excellent articles that explain how the golden ration can be applied to aquascaping to produce an effective composition.
George Farmer has also written an excellent series of articles that cover the basics of aquascaping
(I would post some links, but don't want to start breaking any forum rules about posting links to other fourms.)
However you're already following the most important rule .......
I planted my tank according to what I think is nice
.... it's your tank and whilst we always find ourselves striving to improve our designs (and always should), unless you are looking to enter competitions, what pleases you can never be totally wrong.
Even if that involved day-glo gravel and plastic skulls
Al