I think you need to mix it up a bit ... playing the same key can make you lazy, and you may start to finger the same chords in the same way all the time ... too much chance of monotony, I think.
Certain fingerings "lay" differently and provide a better hand position over "C". As a matter of fact - "C" is not AWLAYS the easiest for lots of chords. The "B" major scale falls so naturally under the right hand on a keyboard - it's much easier to hit the notes than in "C". It smooshes your hand less. There's nothing wrong with using capos or buttons, but it does chisel away at your skills, and reduce your effectiveness when those features are not present. I really like most of the keys up to 4 or 5 accidentals. After that - I loose some dexterity due to thought fatigue!