I haven't found a situation where I'd want to use a different sender address for the notifications, and I always have to leave the form setup to look up the address for FormAssembly's default sender.
To me, it would make more sense to have the Sender email prefilled with FormAssembly's default sender address, but still let people edit it if they need to.
The sender address should already default to 'report@formassembly.com'. That is, if you select a form, go to the 'notifications' tab and enable the email notification, then select 'custom template'. The 'Sender email: ' field should be pre-filled by default.