jcr
If you plaster enough stuff on your guitar to dampen vibration to a significant level, of course it will affect the sound. But the main reason not to do this is that it looks tacky and decreases the value of the guitar. Most people without guitar repair experience don't know how to remove stickers without damaging the finish or leaving gummy residue on there. Even if you know how, if they've been left on there too long, sometimes there's no way to get them off without there being some sign of their presence, because of the area under the sticker having been protected from light. I've picked up a lot of guitars dirt cheap because they were "decorated" by young owners, and made a profit after cleaning off the "improvements", but it's never 100% certain that the job will come out perfect.

exalted wombat
It will affect the sound a little. Not enough to make a real difference, unless you plaster them on three deep, all over! But you surely don't want stickers just for the sake of having stickers? When you play somewhere interesting, and there's a sticker available, maybe consider displaying it.

Mikey, just Mikey
It is completely true that stickers or decals will negatively affect the sound. Don't do it. Instead, give your guitar to someone who will treat with the respect a musical instrument deserves and go get yourself a scrapbook to decorate. Guitars are for playing, not for decorating.

TommyMc
Vinyl decals and stickers have the *potential* to dampen the tone of your guitar if you apply them to the sound-board. The stick-um can also be problematic to remove if/when you realize how tacky they look and want to remove them. Having said that, the pick-guard is a huge piece of plastic which is glued to the top and any adverse tonal effects seem to be acceptable. My advice is to avoid stickers and use water-transfer decals. These will be easier to remove and are less thick.....meaning the sound dampening potential is less. Smaller is better, and if you can find a different location than the sound-board, that's best. I have a vintage guitar from my hippy days with a nice butterfly decal up by the tuners...right under the makers name. It's discrete and in a location that won't affect the sound. If you do a web-search for water transfer decals, you'll find some suppliers who sell special paper so you can design and print your own. Another option to customize your guitar is to leave the guitar alone and get a custom strap......just a thought.