While I agree with the above comments, one thing worth noting is that, like all "luggage", there is a certain element of "stuffability" possible in a backpack, which would not be possible in a bin liner, at least not easily.
You can really jam all the softer things into a backpack (like clothes, sleeping bag and liner, etc), and then cinch it tight afterwards (a good thing of course :)) - attempting the same in a bin liner might just tear it, or worse, stretch it, therefore not giving you any sort of useful indication.
So my gut feel is that unless you have any particularly large or weirdly-shaped items, it is probably an ok way to get a rough estimate. Once you have a backpack, you will always pack to that size anyway: I always manage to fill the darn thing up to capacity, whether its a 1-nighter or 3-nighter :(
As a side note, I tend to pack my clothing in a cheap bin liner first anyway, and then suck all the air out: its amazing how small and compact you can make it, and of course it is mostly waterproof afterwards too...