When you take the cone-cylinder junction as line of support, you can reduce the design length for external pressure and as a result your required thickness for the external pressure for the cone, small cylinder and large cylinder goes down.

But, you have to prove that that junction is strong to provide support, hence there is special Moment of inertia check to ensure that. This may require a stiffening ring at the junction.

If you do not take the cone-cylinder junction as line of support, then the cone is considered as part of the attached cylinders. The design length is much larger. So, the external pressure required thickness for the cone and the attached cylinders increases. But, you do not have to satisfy the moment of inertia requirements.

Additionally, you have to make sure that cone thickness is > required thickness of both of the cylinders.

So, if you have ring close to the cone-cylinder junction or your actual thickness is much more than external pressure required thickness then do not take the cone-cylinder junction as line of support.

I would say try it both ways to find an economical solution. You will slowly get experience with it.

It would be good to know someone else experience with it too.