I believe in cusps, I consider any planet between the last and first degrees has both influences.
I don't find a reason to consider just the sun, I think if you believe in cusps (some don't) you have to consider them in all planets.
A chart flows, the energies don't start and finish just in the 0ยบ, they transform, a slow and constant change, that's why each sign has similarities with the previous and the next sign.