The tooltip for up voting a meta question suggests that it is appropriate when the question is clear, shows prior research, and is useful. A down vote is appropriate when the question the opposite is true. I believe these tooltips are common throughout meta and parent sites.
If there's a meta question that is poorly researched, or unclear, or is not useful, then this is straightforward. However, there is a practice (presumably originating on meta.stackoverflow.com) where these are not the reason for up/down votes; rather they indicate agreement or disagreement with a suggestion in the meta post. I presume this is because meta.stackoverflow.com users are too lazy to add a "Yes" or "No" answer or vote such an answer up or down.
Based on my own experience (I've used Stack Overflow for 3 and a half years, but had limited meta experience before this site) and comments I've seen others make, this is especially confusing for new meta users, who are ones we want to be encouraging.
I think we need to have a policy about this so that everyone either knows what to do or can figure it out by reading something on our own meta site.