It totally depends: when I do it, it's very cool. When others do it it's ugly and childish. I think all of Talkbass agrees with me on this.
It depends. If I had something rare or unique I'd think it un-wise. Or something particularly valuable or expected to appreciate in value. Otherwise, whatever floats your boat.
It depends entirely on context and genre. Actually, they're sort of like fedoras these days: 95% of the time they're tacky, cringe-worthy, and exhibited by people unfit to display them, but the minority of people who pull them off with the whole package make them look cool.
You will have a hundred different answers to the question and they are all correct. Do what you like with your bass.
It is a good way to ensure you can never get more for it if you ever want to sell it. But, just like cars, tats, piercings, image in general, it is your own personal expression, do what you will.
I usually think it's really juvenile and amateurish, but I really enjoy Jonny Greenwood's sticker'd out Tele Plus and Mike Watt's EB3