Try to place turrets in places that don't see a lot of fire, or behind walls.
King's Row: On either side of the gateway between the statue and Attack spawn. Place them far enough from the entrance so Pharah/Junkrat's bombs don't kill them instantly, but close enough to hit whoever runs through
Numbani: Symmetra's not great on Numbani because of the wide, open spaces. There are two turret hotspots here. The first is behind the cars and trucks and the bridge. The second, once you inevitably lose that position, is to the side of the Payload spawn where the chain of rooms leads to the stairs. Place them either inside facing away from flankers or on the outside around the doorways
Hollywood: Two locations here. The first is also behind the cars and trucks just like on Numbani. There's a stairwell right in front of the first chokepoint that's also good for turrets. Near the payload point you should put them on the wall of the building, with the trees. These are almost untouchable without them being in line of sight of your Bastions and Torbjorns and whatever.
Route 66: There's a lot of nooks and crannies between the broken train cars that you can hide sentries in. Again, place them high and facing AWAY from attackers so they're forced to look backwards to shoot them, leaving their behinds open.
Volskaya Industries: Around the large truck at the first chokepoint. You can choose to either lay them around the truck or cut off the rooms to either side of the truck.
Hanamura: Same deal as King's Row. To either side of the chokepoint gate, far enough to avoid explosion damage but close enough to focus fire. You might also lay a few in the room to the left or the entrance to the top right depending on what they're doing.
Gibralter: This one's a bit of a crapshoot, I haven't really figured out good turret locations so I just slap them everywhere on rocks and over/behind doorways
Anubis: The gate has two doorways, I like to put one or two sentries in the middle column, and also a few in the room to the right.
Dorado: I don't have much experience with Symmetra on this one.
Rules of thumb for turret placing:
1) Place them high and place them low, so they have to look around to see all of them
2) Change locations if they're getting wiped out too often
3) Place them far apart so any shot destroys one at most. If a Reaper can kill all of them just by spraying, you're doing it wrong
4) One or two on payloads (on the underside) can surprise Tracers/Genjis
76% win rate as Symmetra, but I play her exclusively on defense.
