I'm doing a bit of RF PCB design, and one of the things that caught my eye is the "Controlled Impedance" option. Checking more boxes always costs more, so I want to know if this is worth the extra money to ensure functionality on arrival. For the RF portion, I'm using 50 ohm microstrip-line on a 4 layer board. (Top layer  is signal, top inner layer  is a ground plane)
Most board vendors have made their laminate stack-up contents and thicknesses available on their website, and I have been able to calculate the width of the transmission line to my satisfaction using their numbers.
- What is the benefit of using "controlled impedance" or "controlled dielectric"?
- At small distances (about 1/10 wavelength), would the impedance bump matter? (I get about 2 ohms difference in Zo from changing the dielectric constant by +-0.4)
- Is this something that should be done for production boards but isn't necessary for one-off prototypes?
- Have you ever used this feature?