Intruder speaks the truth. In classical "critical point / equal time point" calculations, you assume the single-engine over-water TAS as declared in the company Flight Ops Manual. So the considerations mentioned by BIK do not apply - if you were a sufficiently sophisticated pilot to adjust your power setting for best range for separate head or tail wind cases, you would only do it with all engines operating.
The only circumstances using the same flight level on or back that I can see when equitime might not be equifuel would be if there were a difference between the density altitude and the pressure altitude, depending on whether you go on or back. You would fly the same flight level whether on or back, but the temperature, and therefore the density altitude would be different.