Absolutely definitely positively book your seat or you will end up in the middle of the block of 4 in the centre with no aisle access. You mention World Traveller so it's BA? Don't know why it shows a charge as you normally choose your seat when you book, at no charge.
You don't want bulkhead because that's where the people with babies are put because of the cot facility.
1) You want to be as far forward as possible – better air quality and less bumpy
2) At least 2 rows back from bulkhead – screaming babies
3) Config will be 3-4-3. Go for aisle seat, centre block – access to the aisle, and no-one inside you needing to get out (if there are two of you)
Edit – everyone I know prebooks their seats, especially on long haul, so if you wait until you turn up at the airport you will get the worst available