You don't want to run large currents through the chassis for any purpose given the fact chassis is typically made out of materials that aren't great conductors (i.e. steel), plus connections between the components aren't great. For low current applications like blinker / stop lights it may be acceptable. The thing to watch out for is some battery cells have a positive terminal on their bodies, so if the chassis is intentionally interconnected with the negative, chances of a short circuit are increased in case of a collision. There may be other considerations with isolated DC-DC converters. Bottom line - if you're reusing existing wiring to 12V automotive accessory equipment, then chassis may be fine - but that's also where isolated DC-DC comes in, you may want to connect the negative output from DC-DC to the chassis, but leave the traction battery decoupled from the chassis.