8 bolts are stronger bolts and you could use them virtually anywhere, depending on the application, but they are also more spendy. if bolting on a fender there is no reason to use a grade 8 bolt. there is a lot of bolts holding the fender on already so the shear number of bolts is mostly to keep the fender aligned properly and stop it from vibrating and/or flexing which would cause the fender to flex. for suspension items, sure, use a grade 8 over a grace 5. manufacturers use lesser grade bolts when they can to keep costs down. no reason why you wouldn't also. a stainless steel bolt is nice looking but is also soft so it will strip out before a gr 5 or 8 bolt. use these for decorative areas or stuff like a front fender under hood area where they are seen but there is also lots of them so the fender is not gonna fall off or shear a bolt off from stress. also, a big thing for bolts in general is to torque them properly. when a bolt is tightened it stretches, that is why they say to replace engine bolts for main bearings and cyl heads especially, the old bolts have already stretched and may break if re-used and stressed again.
here is a couple of links to bolt grades and a quick explanation on the stresses etc. some cap screws (bolts) have a shoulder similar to a built in flat washer under the head of the bolt and act sort of like a flat washer. on these the underside of the hex head doesn't dig into the surface it is butting up against as badly as some of the cheaper bolts without the raised part under the head. measure bolts from under the head to the end of the threaded part, so the shaft length, not including the head of the bolt where the wrench fits. there are bolt gauges available that have holes to tell the size and rulers on the side to measure the length etc. grade 8 bolts usually run tandem with hardened washers and nuts.
http://rtstools.com/are-grade-8-bolt...s-10-9-metric/
this link also has links for torques etc if you scroll down
https://www.fastenermart.com/underst...d-classes.html