I learned my lesson last year. Last year...I lost 45k words. You read that right. I was only 5k away from my goal, and then my computer does a weird blue screen of death (or was it black? I forgot now, but both happened to me the same year). I could not get them back at all, since my computer wouldn't let me go pass the Windows start screen I was super pissed for about a week...which was all I had left before NaNo was over. I could not, no matter how fast I write, type 50k words within a week left. It was not possible, for me anyway....
emotion_donotwant
I was seriously on the verge of throwing my laptop out of the window....
So yes, it is very important to back up your stuff whenever you can...and everyday at a certain time if possible, or constantly if you want, too.