Whether you need a slider or not will depend on the type of FW you get. Many of them today are set up with an extended pin box so you can maneuver without needing a slider. I have one just in case I get into a real jam and need to back up at near jack-knife angles to get free. To date, I've never "had" to use it for my FW.
However, I did help a friend move a used FW a couple of blocks one time, and it had the old style king pin that you simply couldn't use on a short bed truck. I moved the slider back, and very carefully and slowly drove it the couple of blocks to park it for him.