Those that claim it's better with whiskey are simply referring to the reduced tension that comes from alcohol. There aren't any properties to whiskey itself that is helpful. However, if you think about it, there are some things you can understand about your voice:
1) If this helps, that means perhaps you have unwanted tension in your sound
2) Having to rely upon alcohol to perform well is not a good routine to get into and could lead to other issues
3) Alcohol is a diuretic, and when a person becomes dehydrated, the first thing to be affected are the vocal cords - and this can all happen without even feeling dehydrated.
4) Since I'm really into vocal health, obviously a rough/gruff sound is not something that I feel is healthy production unless you are careful to only use it for effect and not use it al the time. So the above things are only for those who want to have voices their whole lives, but if you prefer the rougher sound, just understand at some point this kind of vocal abuse will take its toll and you'll have a harder time producing any kind of tone.