I have a Reese slider in my 8-foot bed. Its there because I transferred it from my short bed truck when I traded trucks. I did this to save money vs. buying a new hitch.
I have to back at a sharp angle to get my 39-footer into my driveway. I can certainly tell the difference between backing in with the hitch in the "transport" vs. the "maneuvering" (slid back) position. The trailer reacts faster when the hitch is back. Is it a huge difference? - no. But, it helps enough that I always slide it back before backing in. I wouldn't go out and buy a slider just for this purpose, but if you already have access to one, it helps.
Ken