I hope you're kidding.
This "resale value" thing is starting to get a bit laughable. It's nothing but a cover for OCD (that makes owners feel like they're not the ones with a problem), and that it's just to protect the "resale value". All the while the cost of a case ($20-30, and essentially worthless come "resale") gets lost in the math, along with the fact that at MOST a "perfect" used 4S only fetched a tiny bit more than a "functional" one (per eBay, Amazon, Gazelle, Craigslist, your mom, etc...). Not a broken, water damaged one, just a normally used, fully functional, lightly scratched one.
I do agree that a flawless phone WILL be worth slightly more come resale time, but not significantly more than the amount it costs to KEEP it that way (cases/films/pouches/etc.) especially when you compare it to the average non-dropped, non-abused, naked phone.
Naked since day 1. Not a scratch. I do keep it in a sunglass bag and make sure not to drop it (if it happens I'll just get a new one, no worries). Chances are even if it WAS in a case, the damage (even if slight) would bug me enough to get a new one anyway, so why bother.
The "resale value" belief is nothing more than a cover for OCD.
Go naked!