If you want to have a seat that you're happy with, then the best way of securing that is to pay for one if you're not entitled to have one pre-allocated for free. On any particular flight, it's anyone's guess really as to when the better seats will all be taken, because it can be a pretty random process. So there's no way of telling you when would be the best time to do it.

If you think you will pay for one anyway, then there is no point in delaying. If you want to wait to see how many get pre-allocated before the time you become entitled to choose for free (which may not be until online check-in opens, and even then a choice is not necessarily available), then there are free tools by which you can keep an eye on the seat map. Not least of these is by starting a dummy booking on ba.com until you get to the point where you have the option of pre-allocating a seat - that will then show you the current seat map.