I would. Here's a scenario... all of a sudden you develop severe troubles with your MacBook and it's deemed that it needs to be replaced. You put back in the original stuff. The bad MB goes back with the original parts. You get your replacement MB. You can re-upgrade with your upgrade stuff.
Needless to say, this probably won't happen but it could.
You could also get an enclosure for the original drive and use it for backups.
-GDF