here's the difference (courtesy of wikipedia)
- 6 speed - 7.8mm (all brands)
- 7 speed - 7.3mm (all brands)
- 8 speed - 7.1mm (all brands)
- 9 speed - 6.6 to 6.8mm (all brands)
Don't take this as gospel, but it's a good guideline for comparison. You'll note that there's only a very tiny difference between 7s and 8s chains.
There's also a decent amount of latitude, but here are the drawbacks to going off spec. Wider chains are more finicky about trim in the rear, and more likely to brush the outer chairing when coming from the outside rear to the inner ring. OTOH narrower chains can sometimes snag behind the teeth of the inner chairing when downshifting.
So feel free to fudge, and if you don't have problems so much the better. But if you experience problems you know why.