1/4" per foot is standard for assuring that water runs in the preferred direction. Something to help keep wood compartments dry seems like a good idea.
If you're worried about the finished structure being visibly crooked, you could choose to use mortar between the lower courses, making up, for example, 1/4 to 1/2" per course until you get it straight then dry stack the rest. You'd probably want to check with someone who knows regarding how thick and thin mortar joints can be and still cure properly.
You could also probably offset successive courses slightly in the horizontal direction. If you're off 1/4" per foot that's about 1/6" per 8" block horizontally. The offsets would be covered by your finish material. The horizontal surfaces of the blocks would still be a little off, but you can easily fix that when you pour the hearth slab. |