How will you be exploited just because you're playing a certain stake? It is completely player dependant. Yes as you move up people will get better and therefore are more likely to exploit you but at the same time there will be people at 5nl who will exploit you even if it's purely just down to how they play without realising what they are doing. It's important to be aware of but we should definitely deviate when we have the information available to us to exploit people, ESPECIALLY at lower limits because people won't adjust/will adjust poorly than higher up.

The second bit completely depends on the situation. Flatting a hand like A3s in the BB vs a btn open may be +EV, but 3betting it can be more +EV. In which case we would 3bet it because we don't care about +EV lines, we care about the most +EV lines.