First, you must decide what is the purpose for the shot, what is it that youre trying to convey with the photo, then, crop in camera, shoot the image for the planned output. . .

cropping is key to great images. The closer you can come in the camera to your plan, the better the image. If youre shooting covers, leave enough for a header, etc. Your images should have a purpose.

If I'm really in doubt I shoot horizontal and then crop a bit, with 12 mp you can still get a cover out of one side or the other of a horizontal image. . . the thing that takes time and practice is to know what your shooting for and "fill" the frame for that specific output.

Many times if you shoot too tight, you wont be able to crop to your output. . . in this case, the shot may still be nicely exposed but it may well be too large for the output.

so, in my view always "crop" in camera, I know other's that just shoot a bit wider and crop at home, much of it depends on the output and the subject. . .