Beyond that, if your root partition is getting full then you need to do something. If the root partition fills up, really ugly things will happen to your system. This is one of the few ways you can crash Linux, and it'll go down in a fashion that will make a mess of things for you.
Now, to copy a DVD could mean that k3b wants up to 4.4 Gigs of space or so, and if you don't have 4.4 Gigs available on / that isn't a big deal. OTOH, if you only have 500 megs or less available on /, then you could be looking at a problem.
You should investigate. The df command is a good place to start.