For me this is an engineering judgement call a lot of the time as you wont find an example of this in the codes or books.

However looking at it from first principles, keep it along the neutral axis where bending stress is zero. The only other thing to worry about then is shear. You can check the reduced cross sectional area quite easily for shear but for a slab I can’t imagine you’ll have any difficulty in this regard.

Similarly if the slab is a diaphragm subject to axial loads, again you can check this easily but the % reduction in cross sectional area is tiny from what you’re saying above.

I’m assuming above that this is not a PT slab.

I think for bending the impact of the pipes is minimal. Mid depth is probably in the tension zone where the concrete is neglected anyway. For shear, I would reduce the effective width by the sum of the diameters included in the section being checked.

If your slab has more than one span, the compression and tension zones will "flip" depending on where you are in the span (locations of positive and negative moment). Therefore, your pipes would have to be draped if you are trying to keep them in or out of a tension or compression zone. I think your best bet is to keep them in the middle of the slab near the neutral axis.

Alternatively for a single span slab, I would keep the pipes out of the compression block (somewhere between the tension reinforcement and the compression block). If you had to locate the pipes in the compression block, I would calculate the stress "lost" from the compression block due to the pipes and increase the tension reinforcement by that amount, since C = T.

Structural considerations aside - I’m not a fan of the detail for fear that deflection/shrinkage might stress the pipe... and good luck finding the leak at that point!