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 0º, they transform, a slow and constant change, that's why each sign has similarities with the previous and the next sign.
I think it's more relevant for a Moon sign in later degrees than it is for a sign in a new degree. When you have a Moon sign in say...Pisces 29 degrees, you have a bunch of influences at play: 1) Pisces 2) Pisces in the decan of Scorpio (more Mars influence, dilutes Pisces a bit) 3) Pisces in the dwad of Aquarius (just assume Pisces is getting even more diluted) 4) Pisces that is going to progress very quickly into Aries. Signs towards the end of its cycle are trying to grow into the next one...energies seep in.
Essentially Pisces is already heavily diluted as a sign based on all of these other random influences, so any traces of the upcoming sign are more noticeable. Compare with Aries at 0 degree.
1) Aries 2) Aries in the decan of Aries 3) Aries in the dwad of Aries 4) Aries that just left Pisces (hence some residual energies, but since signs progress forwards, Pisces energy is being left behind as the native grows).