[Michmant]

Catnip can be used as a training aid, actually. It is also a drug (on the level of marijuana, but more of a stimulant). These two things are not mutually exclusive.

The first thing to remember with cats: There are no guarantees. Putting catnip on a scratching post can encourage them to use it, certainly, but it really depends on the cat - some will, some won't. My cats all prefer the cardboard scratchers, but they still scratch furniture some. Some cats can be encouraged not to (either through use of sticky tape, or sour apple).

Probably the most important thing to remember is why the cats scratch: to sharpen and shorten their claws. You can help this by cutting their claws regularly (once every two weeks, or so, for my cats). Some cats don't mind it much, some mind terribly - but if you're strong with them, they usually will calm down enough to let you do it by yourself.

I think at the end of the day, though, you just have to balance how much you care that your furniture is pristine, with how much you want to be humane to your cats