I don't know. If i got a signature or sketch i wouldn't want it personalized. Not because i have any intention of selling it but because i would just rather not have it there and yes it devalues the book should that person ever want to sell it down the road. I understand why some artists do it, seeing something you did as a favor for a "fan" and seeing it flipped on eBay the next day for a few hundred bucks would probably annoy me too.
If its a gift, I'm not going to sell it - personalize it. If you diden't charge me? Personalize it. If I paid for it, I should feel ok with selling it later on down the road as it's my purchased property.
My 2