Not as important as YOU
How important are college rankings? It depends on who you are.
If you are a magazine like US News and make huge profits off of those rankings, rankings are very important.
If you are a college, rankings are important to you if your ranking is high or is improving. If you are a college and your ranking is on the low-ish side -- well then -- they probably aren't very important to you.
If you are a high school student, rankings should not be of great concern to you. Sure -- you probably have a few peers who are crowing about how they are going to apply to Stanford because its ranking is higher than that of USC, but really -- are you that shallow?
You -- being the highly intelligent, strong, motivated, and deep thinking individual that you are -- are probably looking at what YOU want out of colleges FIRST and THEN look at colleges that have those characteristics, right? Right!
Besides, rankings are so variable. One ranking's number 5 might be another's number 35. There are so many rankings out there now (US News, World University Rankings/Thompson Reuters, Forbes, Washington Monthly, Parade), you will probably be able to find a ranking to justify your college choice!