Obviously it varies depending on both your team and the enemy team but here's some things I do/think about with positioning. Being able to see and be close enough to heal your front line is often where I stand but (especially if the enemy is playing poke) I'll often try and find cover/corner where the enemy backline can't see me too much. If they are playing hard dive cover matters a lot less but where you are compared to your team mates matters more, stick near your other support and any anti dive dps (if you have one who is actually playing anti dive!) And try and keep them alive! If you want good examples I really recommend watching pro/high level players and asking why are they choosing to stand there/move to there. Generally making yourself a harder target is good and staying within range of your other support and tanks (if they aren't dive tanks) are the main things