Granted, markets don't go up forever and we have seen some price declines in many markets the past year (luckily none of which I am in).
When I look at the data, outside of 2008 there really aren't major life altering declines. Furthermore, even with a price decline of say 5%; the impact on me is nearly moot.
This is because I am holding the properties as rentals for the long-term. If the value of my property goes down in the short-term, well not much changes for me.