Figuring out what settings cause stringing can be a rather mysterious business. Some filaments are also more prone to stringing than others. To help add a bit of rigour to your experiments, here is a test object designed to examine stringing, while also ensuring that any changes you make do not compromise surface finish.
So far I have found that reducing the PLA extruder temp to 220c and setting the Travel Rate to 80mm/s has the most impact, along with using 2mm retraction in combination with 16 steps of deprive, and the tweaked dwindle http://www.thingiverse.com/thing:61022.
Increasing deprime appears to have no effect because the extruder max speed change/acceleration limits how much deprime can occur. If you increase the extruder max speed change, you have to decrease the max acceleration, or the print head goes off the rails. So I tried 40mm/s with 128 steps, but it made very little difference. Another thing that can reduce stringing is turning off the blower fan, but this will cause small parts melt - so its really only practical for plates of large parts.