I'm a little late to the discussion, but there's a few points where I include slopes in my quests. It always feels a little strange to me, and I think it's usually better to just keep the mountain heights even, but sometimes I can justify it. Here's an example.
Here, the sandy ground is meant to be the lowest level, with the grass and lower mountains on the same plane, and additional mountains above them. The slope happens at the point where the grass transitions into the sand. If it were truly a gradual slope, part of the lower mountains near the grass could have a height of one tile instead of two, but it looked too bizarre to do that, so this is the best I could do.
Edited by Aevin, 10 April 2014 - 03:15 PM.