Good point, Dave. The post office reserves the right to use the bottom 5/8" to apply barcode labels. We usually try to stay out of or near the top of the area so a label may overlap but not entirely cover anything printed there.
The good news is, at least in my neck of the woods, ink jetting seems to be replacing labels. If I see one or two labels a month it's a lot. The labels are also designed so they can be easily removed. They're about as tacky as a post-it note. Just MHO, when you're working with a limited amount of real estate, like a 4X6 postcard, while it's not a bad idea to avoid the area, it's of less concern when it comes to filler text or redunant information that's been printed on both sides of the piece.
