Graphic card hangs down in my PC. Piece of wood okay to use to support

If its properly anchored at the I/O panel end it shouldn't sag.

Also, are you certain its fully seated? Most motherboards have a latch that secures the board in the socket.

I agree with fireberd if its properly seated the card should not sag. Your PC isn't going to catch fire, but I'm not sure I'd want to put any pressure on a hd cover. Why not just lay the PC on it's side?

Long graphics cards are notorious for sagging from their own weight. I've seen lots of photos of longer graphics cards that sag. My own card isn't very long and it sags a bit. And I know darned well it is seated properly. I've also seen photos of peoples ingenuity dealing with that sagging. Simplest are wooden dowels going from the floor of the case up to a corner of the card, often using an existing hole in the card to run a screw through and into the top end of the dowel and another screw through a hole drilled in the bottom of the case and into the bottom end of the dowel and fishing line suspending the sagging end of the card from the top of the case. I've also seen elaborate brackets devised to support the sagging cards. Backplates are often used on long graphics cards to minimize sag as well as to look good.

I wouldn't use a HDD myself to support a sagging graphics card myself but, "if it ain't broke, don't fix it".

As far as wood being safe goes, I've seen entire PC cases made of wood and even cardboard. As long as it doesn't contact a hot heat sink, it's safe.