Depressions in lawns, or raised areas, that are really noticeable should not be simply filled in since that might kill the grass that is growing there.
To repair a depression cut the sod in an X and roll it back and fill in with good soil. If the area is raised cut it in an X and remove the excess soil. Then put both back and water well.

It sounds like you are doing pathways in a sense so that you are able to utilize the old grass, until you are able to walk on the new grass. Then when you dig up the next section of old grass to reseed, it always seems to either be too high or to low in relation to the finished product.

I would leave a swath of unplanted soil that I can match exactly when I seed the next one. I always eyeball it when doing the final brushing in with a flexible steel rake.

Thank you both for your knowledge. The idea of leaving a swath will certainly help me with my current job.

I'm lost. You say you are digging up the soil before seeding? How deep are you digging and why?

My advice would be to never dig up soil before planting grass seed. Just level it if it has high spots or fill it if it has holes. Then roll the seed down with a rented water fillable roller.