Great question. As long as the app that you installed is billed from within your Shopify account, deleting the app from within Shopify will prevent future bills from occurring. However, a small number of apps require that you create an account off of Shopify and pay for the plan outside of Shopify. If that was the case for you, you'd need to login to your account from their end and cancel the subscription.
If there is anything else I can help you with, please let me know.