Better to have too many backups than not enough I say! If you want to start deleting off your laptop then either replace that with another EHD or use the desktop. I'd also keep a backup somewhere else ie maybe your DVDs at your mum or friends house.
I have 3 EHDs, one of which lives with my mum and I swap that over every month or so to keep it up to date.

You could also look into online storage such as backblaze too.