Red coins and forced exploration?

I'm making a level that has red coins split between a subzone and the main level. The main level has a boss by the goal, but I'm wondering if I should give the boss a key and make the red coins earn the player a prize or would it just be better to make the red coins necessary for beating the level? Basically how do you all use red coins?

I should add that in the sub zone there's a "mini-boss" who technically isn't suppose to be beaten (the intended route has the player solving a puzzle to avoid the fight) but if the player decides to fight the boss (a bulked up bowser) there's a key that leads to a secret room in the sub-level.

So to sum up, so far I have two keys in my level. One's for hidden room in the sub level and one's for getting to the goal. If I give the boss in the main level a key (a third key) then the level can be beaten quickly and provides little incentive to explore for the other goodies (like the red coins or the secret room). Of course I can make it so that the player has two options for beating the level right? Collecting all the red coins or beating the main boss? Sorry I'm rambly it's kind of hard to explain.