I'm no expert, but I would think that with steep sides the cattails will not get very thick and they would serve as a natural filtering mechanism to all the water that runs into the pond from the bank. Not to mention steep banks could use some plant life for stability. If you get rid of the cattails, what holds the banks together.

I can appreciate your concern for using any herbicides in the pond and I would NOT use anything manmade (or maybe even otherwise). If the cattails truly offend you, say for cosmetic reasons, I would consider removing them by hand and try to reach an agreement with them with regards to how much work you are willing to exert to remove them and how many can stay.

Fish on!,
Noel