Clearing up engy misinformation;
Turrets have a ~120 degree fire arc, HOWEVER they can and will turn 360 degrees when they are attacked.
So if you come up on one from behind that isn't going to intercept your movement, you can just ignore it and it will ignore you too. If this was ET:QW you'd be able to 'spot it' as an objective for your team mates, but TF2 isn't about that level of depth
You CAN place them on awkward to reach ledges AND still have them aiming in the direction you want them to face - right clicking while in 'deploy mode' lets you rotate your SG 90 degrees left or right, so you can get them pressed right up against walls without having to get 'behind' them to do so. It's not quite as versatile as being able to rotate a deployment after placing as you could in TFC, but its better than only being able to place in the direction you're facing in.
You can also do this with dispensers, so you can get a dispenser right on the front lines but without it blocking team movement through chokepoints.
Also, while an SG is being sapped, it cannot fire, so if you're a spy trying to take out an engy SG + dispenser emplacement, your best bet is to drop a sapper on the SG, THEN kill the engy, then finish off the rest of the emplacement.