I started playing with reflections using the old Bryce standby - a couple of spheres hovering over a reflective surface. (I was going to try water later on.) I used the basic reflection settings in the Alternate_Diffuse channel on the Square - Hi Res. I rendered it in firefly. (Picture on left.) The basic settings looked okay, though only 1 sphere out of three reflected. (Maybe the other 2 are too far up, though that seems weird to me.)

When I turned on the Cast Shadow property, I got ugly jaggies in the reflection. (Picture on right.) Am I doing something wrong? I've tried changing the Minimum Shading Rate, etc. Only "Cast Shadows" makes the jaggies as far as I can tell, and nothing seems to fix it. I've tried other combinations, and only the "Cast Shadows" seems to cause the issue, but it looks wrong to not have shadows. Also, why are the other 2 spheres not reflecting? (They're only 1.6 feet up.)

Thanks for any/all assistance!

The other two spheres are in all likelihood reflecting, unless you have the "Visible in Ray Tracing" option disabled in their Properties panels. You simply can't see their reflections from that camera angle; it's out of frame. Try tilting the camera down a bit.

Are you using FireFly's Draft mode or Production mode? What Quality setting are you using for the Reflect node in the Material Room? Do you have the same jagged effect with the other square props?

The image above uses Production mode, a one-sided square prop, and a Reflect-node Quality setting of 1.0.