For BA seatmaps, seatguru is notoriously incorrect and useless. The definitive source for anything BA is TheBASource.
There are certain seats that some people may find worthwhile booking such as twin seats or even single seats (the "throne seat"), depending on aircraft configuration or class of service.
If you have a preference for aisle or window, then you may find the cost of reserving a seat worthwhile. The most preferable seats such as those in the exit row are usually reserved for higher status passengers until or even after check-in opens. Also, BA has recently introduced a theoretical seating policy which favours blocking seats next to high status passengers. These will be blocked until they are needed, so in some cases it may be worthwhile delaying choosing your seat.
Certain seats in certain aircraft configurations have other disadvantages such as IFE boxes taking up half the area available for your feet, so it may be useful to be able to find these and select your seat to avoid them, either by paying or immediately at OLCI.
Other criteria to determine whether paying for a seat is worthwhile, include your height/leg length, girth, preponderance to go to the toilet, etc.
Personally, I wouldn't pay and would be online 24 hours before check-in opens. But I haven't had to pay for seating for a few years anyway.