I have a 6.6 box as well, and a Keystone with a similar nose. I opted for the manual slider. Normally I don't need the slider and haven't had an issue backing into anywhere even when very tight and without using the slider.
Having said that, I've used it a couple of times as insurance when we used to keep the fiver in a lot where the ground was very uneven. Now that it's home with us, that's not so much an issue anymore. I would say get the manual slider, you probably won't need it 98% of the time, but if you're on uneven ground while backing up, that corner may come very close to your cab and using the slider in those situations will let you back up with more focus on the "backing up" part, rather than worrying about the trailer/cab distance because of the uneven terrain.
On mine, with the slider activated, the pin does not hit the tailgate, I think my fiver is high enough over the bed to avoid that... I think it's probably 8-10" above the bed rails.