<p>E3D mentions on their <a href="http://wiki.e3d-online.com/wiki/E3D-v6_Assembly#Usage_Guidance" rel="nofollow noreferrer">own wiki</a>:</p>

<blockquote>
  <p>Excessively long retractions will cause issues by dragging soft filament into cold areas. [...] for bowden systems you might want to go up to 2&nbsp;mm. Retraction beyond 2&nbsp;mm is likely to cause issues.</p>
</blockquote>

<p>I have retraction set to the recommended maximum of 2&nbsp;mm, but I still get a lot of stringing and blobs. My printer is set up with a relatively long Bowden tube (500-600&nbsp;mm). I wonder if I need to push my retraction setting slightly beyond 2&nbsp;mm to take up some of the slack. Is the 2&nbsp;mm a conservative rating (I guess they don't want dissatisfied customers with clogging problems) or is it really the maximum? Is there anything else I can do to improve retraction performance? (I already have a small coasting distance of 0.1&nbsp;mm set.)</p>

Yes, you can increase retraction past E3D's max 2 mm recommendation to compensate for Bowden tube stretch and slop. The reason for the recommendation is that jams will occur with most all-metal hot ends if you pull molten filament up into the cold zone. Any molten filament that enters the cold zone rapidly cools and hardens and sticks to the walls, very often forming a jam.
So, the requirement is to keep your retraction distance
at the extruder
less than 2 mm. Additional retraction travel that is absorbed by the Bowden tube and not seen at the extruder is fine. I personally run 2.5 mm retraction on an E3Dv6 Bowden system without any issues.