It is not always necessary to scale your artwork down but here are a few good things to know

Vector Art
Will your file be over 200″? If yes, you will want to scale it down to 50%. While Illustrator has a maximum artboard size nearly 228″. Acrobat and others will only read up to 200″ resulting in lost artwork. So while the artwork looks ok in Illustrator, it will not print correctly.

Raster Art
Will your file be over 30,000 pixels along either axis? If yes you will need to reduce the resolution of the file to reduce the overall file size to 30,000 or less. This will maintain the correct file size and aspect ratio. Don’t be worried about losing quality. A 30,000 pixel document at 150dpi is nearly 16′ and would not be viewed close up anyway.