I think it's fairly effective, that ledge is a good spot. The drawback is that, as mentioned, you're drawing back as far into your little shell as you can. It's still probably better to keep turrets out as far as you can maintain them.
However, since it's stupid not to put SOME turrets near your reactor, putting them along that ledge is a good plan. I've been experimenting with trying to put structures, like armouries, on the ramp to slow down tyrants, but both games I've tried we just owned the aliens anyway.
Since we kinda delved into the general "line of turrets" discussion, I'll just mention that I find putting them in an arc shape of 3 or 4 turrets works best for me. Humans can easily get back into the base, but aliens have to walk by a line of angry turrets. For extra protection, I add one more turret around the corner.
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The arc shape also lets more turrets fire simultaneously. Since they're not in a straight line, there's no "safe spot" an alien can be where only 1 turret can fire on them (while the rest are obstructed by the turret in front of it). Not that I have a wealth of experience, I just find this works well.