Ashley's answer is still valid. Computers produce small rounding errors all of the time. You have to deal with them. If you are basing your animation on VectorY=0, don't. Check for < 0.1, or something like that.
It isn't an issue for my project, but the vector values can be significantly off.
I'm not using vectors to determine things in my game, but even <0.1 would miss it--it's over 3.