Typically no need to do any firmware changes on FrSky gear unless it is a brand new product that released in the last few months or you are after a special feature that requires a firmware change. (pretty rare)
We usually will recommend you use the X series receivers with the FrSky transmitters as they will allow you to have the model memory locked to a given receiver. This is a nice safety feature. Also, tons of sensors and options for the X series. We do have some die hard folks that love the D series, so the last 2 versions are still available, but they sell pretty slowly now. They are best for folks running the older DJT, DFT and DHT modules.
I had completely re-vamped our receiver section of the website with all sorts of good data, but our site decided to eat it. Very upsetting. I need to do all of that work again.