It all depends on the situation and whats happening in the battle for me personally. Also it greatly depends on what my current strategy or goal is. Lastly, I don't usually keep a turret in one spot for too long, I constantly am setting it up at different spots.
Sometimes If I have the upper hand as in the higher ground, I place my turret to the side of the enemy or behind them. Many time though, I would not place the turret right next to the enemy or right next to me ( this puts in in the line of fire ). This is all for my missle turret though. For my Flamethrower turret, I usually place it right next to the enemy and create fire explosions and what not while using it to distract alot. I also like placing it next to team mates who are down or for support
This is all for my Female Quarian Engineer though so a Geth turret may have bunch of other uses

Turrets have so many uses I just find it hard to pin point exactly how I use it, because theres a million ways

For me it all depends on different builds, strategy, whats happening in the present moment and just creativity!
How do you use yours? 
:happy: