+1 to glentiaw
imho, stickers on ur guitar expresses something about you as a person. just the same way as music does, only difference is the communication medium. in any case, i think sometimes, stickers can "enhance" the look of a guitar, moreover, its your guitar anyhow, if u like it that way, do it.
mentioned earlier, abt the musician respecting his instrument etc, i agree, but if respect = dun knock holes in it, don't do anything to personalise it, then respect is kinda boring isnt it? after all, music is about expression, the look of a guitar is about expression too. in my opinion, u can stick those stickers if u feel comfortable with em, u just don't go and intentionally bash the guitar upside down.