Backup before downloading Aperture?

I recently purchased a Macbook Pro Retina and have been using iPhoto and Canon DPP to process my photographs. However, I now want to purchase Aperture and was wondering if I should backup everything on my computer to an external hard drive before the download, or doesn't it really matter?

I have a Seagate 500gb hard drive which I used on my PC for backup, but will have to format that before using it on the Mac.

Any advise would be very much appreciated.

Alan.