In the US there is a general principle of cutting your teeth with weapons in the front seat "under supervision from the rear seater, and then graduating to the piloting and supervision in the back seat for operational flying, see Apache Warrior for an example. Both crew are trained to fly in both seats for currency though.
In the UK the opposite is true where generally you cut your teeth flying in the back assimilating experience and then graduate to front seat weapons and aircraft command. That said the rank structure of the CTR course will also influence this with field officers (potential Sqn Commanders) filling the front seat, SNCO's in the rear seat until the middle where it is a bit luck of the draw; there are the same number of front seats available on a course as rear seats. Once operationally qualified you become seat specific until gaining an instructor rating in one of the disciples and then you can fly operationally from either seat; largely based on greater experience. You can also undertake seat conversion training in a squadron to ensure a particular squadron has balanced crews as people move on. Aircraft command although biased to the front can be undertaken from either seat and officer career path generally means they convert to the front quicker of they had to fill a rear slot on conversion course.