Back in the days when I was fiddling with these options, my rule of thumb was: scan with high resolution, reduce afterwards - although I left the reduction to a picture editor (PaintShop Pro), not the scanning software. Though this may have also been influenced by the tiny little issue that my first scanners didn't support picture reduction.
Anyway, that strategy that seemed to help with Moire patterns. When I got Moire with scanning at 300dpi I moved to 500pdi or even 1000dpi, creating a monster-sized file, and then let the picture editor do its magic afterwards.