yes, 8GB is a fuzzy figure it seems. If you are not buying brand-name cards you may want to ask for "real full capacity" and "taiwan chip" when you buy the card.
We use the smallest 8GB card we have been able to buy to create the images, an old class 4 by Kingston. The 8GB cards we include with JeVois turnkey kits are bigger than that so flashing the software to those has been no problem, at least until now.
I tried a while back to create the microSD image using a loop device as opposed to a physical card but there are some hurdles to that having to do with creating several partitions over a loop dev. We will look into it again as this would allow us to just set the size to 7.5GB or similar. We will also try just chopping off the image a bit as we read it back from the microSD. Since the last partition is not anywhere close to being full and the files are just written once to it with no editing and moving around, it should work fine.