I evolved all the Pokemon I used for battling to their final stages in Y, but in terms of my living dex, only one will be fully evolved to final stage, the others will stay in their pre final forms forever.
In terms of shiny pokemon, I rarely evolve the first one I get, usually I evolve the second one or third one.
Though pokemon that evolve via stones probably don't always need to evolve in order to be better.
In Black 2 I have a Magmar and a Magneton I don't plan on evolving just because I like their middle evolution designs, aka original final evolutions, better.