Quantity of tickets available in a bucket is never going to help you judge whether to pre-book seats. You could have a flight that's half empty full of silvers/golds who have booked up every other seat.

The only thing that can help you there is determining how important it is to you to sit together.

If it's essential - always pay
If it's a nice to have - watch the seat map and if it starts to get busy: pay - otherwise check in at bang on 24 hours out.
If it's not important - don't worry about it.