If you are investing money for the long term, you're going to tap it way down the road. So what happens today is pretty much irrelevant.
Even if the stock market goes in the toilet because Washington can't get its act together, you benefit way down the road by keeping your retirement money in the game right now.
So don't make a decision based on what's happening today if you're going to use your retirement money 20 or 30 years down the road. The things you really have to worry about aren't the artificial crises of the fiscal cliff; they're the unexpected, things like natural disasters, or acts of terrorism, or a brutal world war.