First off, you definitely want to change the WiFi password from the default, and while it certainly is convenient to have it attached to the router, there are still ways you could make it difficult for someone to read it without your help. Of course, for some people, their own handwriting is security enough.
On the one hand old-timers like me are going to question the wisdom of writing the WiFi password on the router (like attaching password PostIt notes to your computer monitor), but on the other hand if the password is unreadable it probably doesn't matter where it's written.
Is putting the WiFi password on the router a thing people do? Are they just leaving it with the default password that's already pasted on the device? I was dragged kicking and screaming into the WiFi age, but I certainly don't have the password on the router (although I can see how that might be convenient, like a computer password stuck to the side of the monitor).