Bleach can and will pit stainless over time. Furthermore, since most kegs are type 304 stainless, the heat affected zone around any welds can become sensitized, leading to higher potential for pitting and stress corrosion cracking in chloride solutions. Star-San or iodophor are better, but should still be rinsed after use.
If the keg is for use as a mash tun or boiler, why not just clean it well and give it a good boil with water before use? That should sanitize it plenty well.