My driveway is sloped at least 5 degrees, maybe more, and it was fine. I used chalks behind the rear wheels as I jacked up the front, (I back down the slope) safety is the only important consideration. I'm not sure that jacking up the front per the instructions is really necessary, but it does make it easier to access the front links. Now that the truck sits higher with the engine off, I can make minor adjustments to the front links without the jack. You will have to get on your back to replace or adjust the rear links, there is no way around that, and it is easier to access while you still have the front end on the jack.
The ride height will not adjust until you remove the jack, start the truck, and drive around for a minute, so don't even bother with any measurements until you take it around the block. And yes, I would not adjust the links with the engine running because the computer will be live.
I've made three 500 mile round trips to Houston in the last two weeks, and the truck handles no different than with the stock links. I've also towed a 20 foot sailboat 80 miles round trip and it was just fine.