Frankly, I never thought about this. I build as true as practical, and with the most even tension the materials allow. I've never used a tension meter to measure comparative tension, so I can't give you a number.

Based on experience, I'd venture that keeping tension within 10% is relatively easy on modern rims (it was rougher when joint distortion was a common problem), so there's no reason to think about what the worst case limit would be.