Also, be aware the gigabit ethernet does use all pairs. If you liven up the unused pins for your 10/100 (especially at higher voltages) dont accidentally plug it in to a gigabit port. Damage to the port will likely follow.
If you read the PoE spec, you'll see that the power is sent on both wires of at least two pair, and because the electrical spec calls for transformers on the receive end, the power will cause no issues. Since there's no voltage difference between the wires in the same pair, there's no problem with core saturation. For non-PoE devices, there's no difference. For PoE devices, the receive transformer has a center tap to access the power.