Need for pads/blocks with hydrolic levelers?

I use the yellow/orange leveling blocks under my jacks. I carry about a dozen. I almost always use them under my jacks, on any surface. Just helps spread the weight...especially if you're on soft ground. But I've seen divots on asphalt and I've seen a jack foot get "glued" to asphalt on a really hot day (had to use a crowbar!) Also I think it's just courteous to RV parks to use them so there will hopefully be no problems. If I'm way off level, I put several under the place that is really low so I don't have to extend that jack to the max.

As to your last question...different coaches, different instructions. Read your manual. On my coach, it instructs to level before extending the slides...then bring in the slides before retracting the jacks.