The 90 degree turn is (as stated earlier) required when on NAT tracks, otherwise a 45 degree turn for one minute to get away from the airway is not a bad idea.(Todays navigation is deadly accurate). The other reason for doing so is to avoid negative G's when starting the descent. As for me I would descend at the
Vb (rough air) speed since it gives the best protection from high and low speed buffet, and one never knows what the turbulence is like at lower altitudes. On the A-320 it's 275 kts which at m0.78 occures at 33,000 feet.