It's always a good idea to have a collection of nuts & bolts on hand for bicycle repairs. Small parts generally come in either stainless steel, galvanized steel, aluminum, or if you have the money to spend, titanium. Titanium bolts were really popular during the titanium parts explosion in the 1990's, as cyclists clamored for any way to shave a few ounces of bike weight here or there. The thing is, we're really talking at the very most a gram or two, or more likely parts of a gram weight savings. Not only is the weight saved using titanium bolts miniscule, but the installation and maintenance of these bolts requires anti-seize compound. So stick with stainless steel. You'll find that over time, the nuts and bolts on your bike will begin to rust or decay with weather. This could prevent a safety issue, should the head of a bolt break off. There is a more common reason cyclists replace nuts and bolts, though, and that is the look. It's not considered good bike maintenance to overlook rusty or worn bolts, even though functionally they are operable. Call it an aesthetic. Hex head bolts in particular are crafted so that the hex indentation is a virtual belly button for collecting dirt and grime. If you let hex head bolts go uncleaned for too long, you run the risk that the hex head shape will give way when you try to turn it with an allen wrench or hex key. So keep those bolts cleaned up once a year or so, and if need be, buy new nuts and bolts as needed, not oly for the safety of you riding your bike, but also to keep your bicycle looking sharp.