Are seats in the back on a B747 better?

I've sat in those seats on Cathay Pacific quite a few times, they are popular with some people, but Cathay shows seat availability and they are often available. If you get those seats at least two rows behind the last row of three seats, the window seats have enough room along the window to stretch your legs out between the wall and the window seat in front of you.
What I don't like about the window seats in the back is you're a little too far from the wall to lean against the wall to for sleeping, but for many people that's not a consideration. Otherwise, I think they're good seats.
Like all airlines, Cathay does overbook, I was once offered a next day business class seat and $250 if I would volunteer to take a flight the next day when the flight I was on was overbooked. I would have taken their offer in a heartbeat, but had a schedule in Hong Kong that I couldn't change.

this may be just my perception, but back seats in the tail of any plane seem to make me feel the bumps more. I like to sit over the wing (if in Economy)....of course, upstairs/forward on a 747 are the best!

and yes, any overbooked flight, they have to try to get people to get off voluntarily first.

Before airlines started stuffing the plane to the roof the seats at the back of the bus were the ones to get when flying long distances. The reason for this is that the airlines never filled these ones because they usually didn't recline that far or at all and they were by the bathrooms. These seats were usually left for the cabin crew to sit in on long flights and if you were travelling alone you got the whole row to stretch out in and sleep.

The airflow in all planes are from front to back so the seats at the back get lowest oxygen content and you'll be more prone to headaches at the back. This is why they seat frequent flyers as far forward as possible to give them the best possible flight experience. The difference in oxygen content can be as much as a factor of three between the front of the plane and the back. If you are feeling as if you are having a headache or are a little ill, ask the flight attendant if they can improve the air circulation, they are only allowed to do so at a passenger's request as it costs the airlines in fuel economy to increase air flow. In general, the seats at the back are the least comfortable, it's noisier as there's more turbulence over the hull and you're getting less oxygen in the air but you'll be likely to have more space and you'll be seated first as they will seat the plane back to front which means you will be more likely to find a place to store your carry on.

All airlines will offer various incentives for volunteers to give up their seats when overbooked but this won't happen till the last minute at the gates. If you're looking for a deal and don't mind waiting for the next flight, let the agent know at the gate. I've often told them I would be willing to give up the seat for an upgrade, an aisle seat or a less packed flight, they'll run through their standard practice of offering a travel voucher first before trying any special requests like that but when they do offer the upgrades, they will include the vouchers they offered earlier. The busier the airport, the more likely that you can volunteer to surrender a seat.