alot of drummers use their china with their snare on their snare hits, for example, Chad Smith (Red Hot Chili Peppers, some of his live shows when they do Can't Stop and when Anthony sings "can't stop" he hits both twice) , Scott Philips. (Alter Bridge, hear intro in Open Your Eyes) This adds effect to their snare hits, and really it's all about effect, as mentioned earlier. splashes are the same, but you can really capitalize on the sound. crashes are to make an impact, or like a bang, but splashes can sound like they're in the background or they're at the side cos of that small sound. some drummers play choked splash hits, for example Branden Steineckert. (The Used, he hits the snare and the splash then immediately chokes it, hear Blue and Yellow somewhere in the verse, and The Taste of Ink when Bert sings "bright-lit city") You don't need splashes or chinas, sometimes it's not always the more the merrier. it's what you want to be able to sound good. what's more, there isn't a definite way to use cymbals, do whatever you want with them. it's your style.
just what i feel.