On episode 11 I had a viewer post a question about stacking ND filters and I wanted to address that. Really, this post could more or less apply to stacking almost any type of filter.
To begin with, the viewer asked about stacking potentially three and wondered if getting filters with a larger diameter than the lens would be worthwhile. In my opinion, the short answer is yes, because if you're shooting with wide angle lenses and stacking filters that equal the thread size of those lenses, then vignetting could occur. It speaks for itself, but larger diameter filters could help avoid that problem. However, there is a longer answer as well...
The vast majority of materials I've researched and even some advice I've gotten from more experienced photographers is that stacking several filters is generally not a good idea. Some of the downsides include loss of image quality (depending on the quality of the filter(s) this could vary from slight to very noticeable) to getting unsightly internal reflections due to the addition of reflective surfaces (e.g. ghosting, flaring). If you decide to invest in higher quality filters, then usually these artifacts are minimal, but costs can go up quickly.
For interests sake, I would like to present a couple of suggestions that could be considered. If the desire is to block out a lot of light, then perhaps getting two very dark ND filters may work. At least there's one less filter to help avoid some image quality issues, and you may get away with using smaller filters which are usually less expensive. The downside is that if you ever needed a lighter one(s) or you still have the need to stack more than two filters, then you either don't have one or have to buy one anyway.
Another option is to get a couple of polarizing filters and stack those. By rotating one over the other you can vary how much light passes through. Here's a more detailed article on this topic: http://digital-photography-school.com/create-your-own-variable-neutral-density-filter
So I hope this helps make a decision for those looking to stack filters. Sometimes too this comes down to personal feelings; some people are against it and say it should be avoided at all costs, other people love it and say it can add another level of creativity and photographic options, even if some cons lurk.