I knew someone when I was in high school who said that if you have a fall back plan, you are sure to 'fall back'.
Here's my philosophy: You should have a fall back plan! If plan A in life doesn't work out, by all means, have a plan B. Because if you don't, what's out there? You no longer have plan A, you no longer have plan B - after that, it's simply a 'what can I find situation'. What do you think?