Just before harvest is there an advantage to only giving the plants 8 hours of light for the last week or so. or is it just a myth?
The advantage that you would be looking for by doing that is swelling up your nugs.. you know how when flowers aren't done they are kinda unswolen and pushing out new growth? Well, shorter nights make the plant think that winter is near and they try to ripen more than they try to grow.. To be honest I turned my room this last week of flower to 10 hours on.. but 8 would just make them swell more so.. that's the only advantage I know of.
It might make them mature a very small bit quicker but I would think it would have an adverse effect on yield. Honestly it probably won't change much either way. The only time I ever went below 12 hours of darkness was on certain sativa-leaning plants in an attempt to get them to flower a little quicker.