If you forget the password, you'll be locked out.
If you forget a password there's nothing you can do. You're locked out.
The situation might not be too drastic, depending on which password you've forgotten. If it's a network password, the administrator can reset it. If it's the password for a Web account, most service providers will send you an e-mail message with the password or a reminder. But if you forget the password to open a document, you're locked out until you can remember it.
Some people say to never, ever write down any password; others say to keep the list in a secure place that only you know (either hard copy or in another file on your computer, for example, in an e-mail message). No one suggests writing passwords on a note and sticking them to your computer monitor.
It's a matter of common sense; base your decision on exactly how important the information is that you have password-protected.
It's up to you.