First off, I've got to say that you probably don't need a battery grip right now, unless you're a pro needing to shoot for long periods or have very large hands...
However, as your dSLR gets a little long in the tooth, its original Li-Ion batteries will die, and eventually it'll be difficult to get replacements, and this is when you'll be glad you've got a battery grip so you get use AA rechargeable batteries.
You don't need to get an OEM model, but they can be more reliable, just make sure you've got one and you store it (and the AA adapter) where you'll find it in the future. I know a battery grip means that my Canon 350d / Rebel XT is still going strong at 10 years old...
Choosing to use Back Button Focus (BBF) is a bit like when you were deciding which camera system to go with - once you've made your choice it's best to stick with it or you'll end up with all sorts of disadvantages in the future.
BBF separates the focus and shutter actions on your camera - the shutter butter now only fires the shutter, and the button on the back of your camera activates the Auto Focus. This can be incredibly useful at times, especially with the focus and recompose technique, and save you a bit of time, but I have to admit I never made the switch.
Give Back Button Focus a go - you made think it's not for you or it could change the way you shoot forever. What have you got to lose?
Jon Ortner is a master photographer, his acclaimed work includes images from Nepal, Cambodia, India and of course New York City.
Jon's latest book, his sixth, focuses on the human form - the nude bodies of Dancers, Athletes and Gymnasts, all created using Large and Medium Format Cameras.
143 Beautifully put together Tri-Tone black and white photographs are assembled in a 176 page hard cover book, complete with slip case and 3 gate folds.
If you enjoy fine quality black and white nudes, or know someone who does, you should take a look at Jon's book and invest in some fine art photography.