Regarding files downloaded from the Internet, the User's Guide says, "If this box is checked (Intelligent Stream Scanning), files that are downloaded are scanned almost in real time. The pieces of data are scanned as soon as they arrive and the next parts are downloaded only when the previous parts are verified to be virus-free. If this feature is disabled, the whole files will be downloaded to a temporary folder first, and then scanned."
If Web Scanning is set to "Scan all files," it sounds like Avast
should automatically scan all downloaded files in one of two ways. Better safe than sorry though if there is any doubt.