That rather depends on what scan you are running, as far as I can recall the only scan that downloads some virus signatures (and not sure if it is downloading full database), that is the boot-time scan. This scan is somewhat different as is user initiated not automatic.
If you have the computer connected to the internet and you have the Virus Definitions update set to Automatic (default setting) and you are also getting the streaming updates (roughly every 3 minutes when you are connected snd using the computer).
So the virus definitions would/should be very up to date and not really need to update before a scan unless the database hasn't been updated for some time.
Check out the virus defs folder and you will get an idea of how frequent/recent they (regular and streaming updates) are. You will see regular VPS Updates and ........_stream, there are currently 251 mini updates in my current stream