You can choose to use what bosses give you - or not. So killing them won't automatically make the game easier, in fact killing bosses triggers newer and harder raid events, and some allow harder/later biome mobs to appear in meadows/black forest at night.

If you really want to keep the spoilers down then check out the stone monoliths you were dropped off at when you were first dropped off in Valheim, then you'll at least know how many bosses there are.