The recommendation is mostly so people buy the correct aftermarket stuff sack size. If I go to buy a stuff sack (we have, me and my partner, a combined 7 or 8 sleeping bags), I actually test the stuffing myself.
You can follow the recommended volume and try to buy a stuff sack with the corresponding value but I found that the listed stuff sack volume is usually uncompressed, so it is its maximum (I have the Outdoor Research models in mind). If your sleeping bag has an advertized stuffed volume, this makes things slightly easier but if you look at MEC's specs for sleeping bags, here a -12°C Western Mountaineering model, they recommend a 20L compression sack for a 8.1L compressed bag. That's most probably too large a size.
For down models, I usually try to aim for somewhere between 1.5 to twice the volume. No more. Synthetics needs a bit more volume for the initial stuffing before compression. As long as it is not overly hard to bunch the sleeping bag in the uncompressed stuff sack, it will work just fine and you'll get the best compression possible. With a too large stuff sack, you'll bottom out the straps before you're done compressing. For example, I have 10L and 15L OR stuff sacks where one of my sleeping bags fits perfectly in the former while I can't completely compress it in the latter. The bag probably compresses to 5-6L.
I try to squeeze until I can't possibly cinch the straps anymore, even using my full weight. This works fine with my oldschool, heavy, and bombproof stuff sacks but with the ultralight models that are most common now, I'm always in fear of ripping it apart, which I've seen happen countless times when I worked retail.