I have just done this. Cancelling your Google Apps account is a deceptive process though, as it requires two steps to be successful.
First, cancel your Google Apps subscription (even if the account is free) on some settings page (billing). This logs you out of Google Apps, and makes it seems like this is equivalent to cancelling your account.
Second, log back into your Google Apps account. This prompts you to accept TOS for some reason. Now you can properly delete your account at the very bottom of some settings page (where you enter secondary email address and make other general settings).
I cannot comment on interruption of service, as this was on a new app I hadn't used yet.