The decision is up to each individual builder...
A CS4-4 will fit in a standard dimple but only the very bottom of the rivet head is actually in contact with the skin... there is a slight gap between the perimeter edge of the rivet head and the dimple.
Paint tends to not want to bridge this gap well, and the rivet head will be slightly above the point of being flush compared to using the 120 deg dies.
The rivets do aesthetically look better when the 120 deg dies are used.
There may also be a slight strength difference because of the ability for the rivet head to tilt slightly in the dimple, but I have not done any tests to confirm that.
Any opinions expressed in this message are my own and not necessarily those of my employer.
RV-6A (aka "Junkyard Special ")