Ideal approach to level a 6x8 ft bathroom?
Leveling compounds are a last resort. They're a pain to work with and make future work more difficult. The right way to fix this is now, while you have the framing accessible. In a room that small, an inch out of level is certainly significant. You'd feel it every time you walked in. 

Either rip strips to stack onto your joists, or sister 2x4s to them with their top edges at level. The latter is simpler if you would need tapered strips. Set the outer ones first and use a straightedge or dryline to set the interior ones. Use heavy-duty construction adhesive and screws for everything, including the subfloor. 

3" screws are probably overkill for your subfloor. 2" or 2-1/2" are fine if the joists are solid. If not you'll get thread stripping and need longer ones. You should be able to drive the heads slightly below flush.