In my high school and college math classes, I've always seen graphs drawn with either no arrows on the ends of the axes or with arrows on both ends.
A colleague recently argued with me that this was crazy and arrows are always placed on one side of each axis to indicate the positive direction. Like so: 
Which style is more prevalent?
On
In my engineering classes, when there are many axes, (for the multiple coordinate systems) the arrow always points in the positive direction for that axis. It would not be easy to indicate which direction was positive without this standard. So while it may be unnecessary for elementary graphs, being consistent helps students learning. So regardless of which style is more prevalent at a particular grade level, to be consistent with advanced coordinate systems, the style where the arrow indicates the positive direction should be used.
I've always assumed arrows indicate the positive direction.
Wikipedia is not as reliable in these really elementary articles than in more advanced ones, just because all of the millions of people who see such topics in school want to contribute and experts may find it boring to look at kindergarten-level stuff. Wikipedia should not be considered authoritative, even though it's often an excellent starting point.