I've booked exit rows for years on BA (rows 9 & 10 on A319!!) and to date, the sky hasn't fallen in
AFAIK.
Contrast with China Eastern, who reckon you have to wait until the gate to be assigned exits.
And with the shift towards online and kiosk check-in, it's all going to fall to the gate agent anyway, irrespective of whether exit row was allocated remotely at the time of booking or on a little machine in the check-in hall.