Yag (plains boss) gives you an item that, while not required, will make the mistlands a lot easier. I highly recommend it. This boss can be boiled down to "do you have fire resist potion". Its fairly easy if you have the sense to clear out the arena area ahead of time.

its a bit of a design flaw, but multiple bosses are totally optional right now.
plains boss: skip if you happen to score enough metal another way to do so.
forest: skipping makes life difficult. It locks you out of tons of iron, literally twice over.
swamp: skipping is harmless, but you will miss out on some loot and a very slightly easier time finding silver.
mountain: can not skip. arguably the best anti skip design.
plains: skipping makes life very difficult.
mistlands: can delay until next biome is ready. Drops nothing of value but boss power is useful to some builds.