But trying to separate your eggs and you get some yolk in your white...that's just really bad!
The reason behind this (only if you are planning on whipping your whites) is that any bit of fat in the whites will make it impossible to inflate them...that's why it's imperative to start with an impeccably clean bowl (if you are doing a big job like meringues or pavlova, wipe down the inside of the bowl with a bit of vinegar on a paper towel...that will do the trick).
But the problem begins when you are separating the whites using the "shell method" (passing the yolk back and forth between the shell halves letting the white drip into another bowl). If you hit a jagged edge of the shell, the yolk membrane will break and then you're in trouble.
Solution? Break the egg and drop the yolk in your hand...let the white drip through your fingers into another bowl. There aren't any jagged edges on your hand to get you in trouble (at least there weren't the last time I checked!)