Ahem ... maybe not one of my better posts ... but, nevertheless, a pretty long-standing rejoinder to the notion that “there will not be a significant number of down-votes.” This is not the case, and it therefore should enter into your plans for your system.
Within ten minutes of the posting of almost any comment bearing my name, I will see Reputation -n, where n varies between 1 and 7. (This post, of course, will undoubtedly garner a higher count.) Over the course of the days that follow, the reputation-tally will usually turn slightly positive.
A simple Google search on the term downvote AND forum is actually quite interesting. There is a lot of human psychology at play here, and if there’s one thing that the Internet cultural experience has taught us, it is that human beings are far more competitive ... and hurt-able ... than they care to admit. Most people would not put up with the things that I choose to. Your voting strategy will have a subtle yet profound impact on the
profitability culture of your site, and you should consider it carefully. (Many people would never dream of putting up with the things that I do, for any length of time at all.)
I remain persuaded that your forum need only have an easy link: “Did you find this post helpful?” And then, to include a minimum upvote-count as part of the available search criteria. When searching for relevant content, I frankly don’t care how many people pissed in a person’s campfire: I want to know how many other people found it useful. I do not want the numbers to be totaled or otherwise mixed, and I do want the ability to be able to filter by that (positive-only) number.