Trainee instructor courses make a point of stressing that turning final & opening the airbrakes should be two separate actions.
2 reasons are usually given:
a) low airtime/un-current pilots have enough difficulty making a controlled, co-ordinated turn at a constant airspeed without adding the complication of opening the airbrakes.
b) to discourage 'landing lever syndrome', whereby pilots get in the habit of automatically opening the airbrakes as they turn final, a habit which occasionally leads to undershoot accidents. Accident no.11 in the latest S&G may be an example (story
here).
So, yes it does apply to all glider types, but the slight increase in stall speed as the airbrakes are opened is not really a factor. It's more the result of years of accident reports where the narrative says something like "I turned onto final approach and opened the airbrakes . . . by the time I realised that I was undershooting and shut the brakes, the glider was too low to reach the airfield".
Instructors are taught to teach pilots to turn final, get the glider settled down in a stable approach and not to open the airbrake until they're on the 2/3 airbrake approach path. It's just one of those things, however safely you do it opening the airbrakes while turning final is considered to be a bad habit to get into.