I'd have to agree with Dave. In the case where you have positive pressure, it doesn't matter if nothing's protecting your hard drives as air will be flowing out those holes on the bottom, not in. For my own case, I have no dust issues other than having to clean the filter and panel in the front intake section.
If you have your case set up where your have negative pressure, for example, if you have a big 360 rad in the front set up with fan profiles, while the top of the case and rear of the case have "unhindered" fans, then I would look to adding a filter below, definitely.
Speaking of filters, has anyone been tracking any dust issues on the PSU side? I have to admit I have ignored it completely, but my PSU fan doesn't actually start until the PSU hits a threshhold, so unless I'm running benchmarks/stress testing or heavy gaming, it doesn't turn on.