I just found Conky today, but I found it flickered a lot, even with -b. I'm using it as a dock with notion (http://notion.sourceforge.net/).
I cloned the Git repo and added a delay after swapping buffers (attached). It goes blank every time xdbe_swap_buffers is called from main_loop (due to the window needing to be resized). Since the text keeps changing, this happens a lot.
If I set fixed_size = true (commented out in patch) then the flickering goes away for me.