I actually use same username and passwords on some of the first sites that i signed up. Then one time it come to my senses that, what if i get hacked. All my accounts would be affected, an it wasnt good. Then i tried to change everything, starting on my email. And now i can say i can make a book out of my passwords and username. lol. Its better to be safe than sorry.
The most important thing is not to lose this book. It is mandatory to have several copies of such information, and the most important passwords must be in paper form.