I have noticed some roundabouts are tagged as “Junction = roundabout” and “oneway = true”, while others only have “Junction = roundabout”.
Strictly speaking the junction tag should be enough, but I can see the map rendering do not show the direction in that case.
Example without oneway setting:
http://www.openstreetmap.org/browse/way/78247078
Example with oneway setting:
http://www.openstreetmap.org/browse/way/328618
Which is correct?
http://wiki.openstreetmap.org/wiki/Tag:junction%3Droundabout
junction=roundabout is implying oneway=yes.
So the oneway-tag is not needed.
I guess Mapnik usually doesn’t want to draw direction arrows for roundabouts because the direction is always the same for a specific country and can thus be omitted. Yet you should always draw the way in the direction of the roundabout’s traffic flow!
When the arrows are missing it is not very easy to spot an error.
The arrows are visible in any editing software (JOSM, Potlatch etc). And keepright (http://keepright.ipax.at) will flag roundabout errors.