Well, what we are talking about are two distinct services really.
Aerating is something that should be done, regardless of how you seed.
If you broadcast seed, it should be done at the time of aeration (Although the germination rate on this type of seeding in established grass is so little, its almost not worth doing).
If you Slice Seed, you should still aerate as normal, but its not critical that its done at the same time.
I like to aerate, wait two weeks for the holes to have a chance to heal a little, and then slice seed with an overseeder.