1. No, unless required for performance. It reduces passenger comfort (APU bleed air smells because it takes in engine exhaust air...)
2. In B738 gain is about 700-1300kg when taking off with engine bleeds off.
3. Don't know. Unless on wet runway, reverse thrust is not calculated in the accelerate stop distance.
4. Yes. Take care with crosswind and be prepared to reduce/cancel reverse thrust when directional control becomes difficult.