I try not to take away from the images by placing a watermark across the middle of the image, but I try put it off to the side where if someone did try cropping it off the image wouldn't look right (eg. cutting off tyres, wheels, heads, legs, etc.) I know people can still copy and paste and edit to get rid of the watermark, but nothing is 100% safe if someone wants it bad enough. If the image was an absolute perfect image, then I would be more inclined to watermark straight across the picture.
I do photography as a hobby, but may earn a little bit of money off of it one day.