Should I use registered RAM (RDIMM) instead of unbuffered RAM?

Yes, go for it. The extra clock cycle of latency from the register is measured in nanoseconds and you won’t feel it in practice — your CPU is waiting on storage, network, or user input far more. Your Supermicro X11SSH‑F supports both types and will auto‑configure. Memory bandwidth and capacity matter more than that tiny latency bump for almost any real workload. Registered modules use slightly more power, but on a server board you won’t notice. Buy the cheap RAM.