You can go into it really deep if you wanted, like 309, 316 stainless etc refers to the ratio of the metal alloys within, theres even an L version 316L, which is a low carbon alloy designed so that it can be welded, as standard stainless the chromium gets forced away from the weld with the heat, leaving a carbon rich area therefore very brittle.
The tensile strength of a bolt is very important, if you used too big a diameter bolt then odd as it sounds it could well be more likely to fail, Ford found this out with engine mount bolts, which is why they now have a 'wasted' thinner section in the middle, a bolt under the correct tension is stronger, if you replace a steel bolt with a stainless, you'll affect this and the fastener may not have the holding power as the original.
Ok I'm getting silly better shut up now.