What happens if I don't choose a Stripe account for something?
Anything left unassigned — a product, a campaign, a permission slip, or one of the dues/donations/subscriptions settings — simply uses whichever Stripe account is currently marked as the school's default on the Admin/Settings page.
You can change which account is the default at any time. Doing so only affects things that were left on "Use Default Account" — anything explicitly pinned to a specific account keeps going to that account regardless of which one is default.