Wires for things like computers, cd players, whatever, can be held up against the wall over a nail, or they can be covered with wraps that you can get at hardware stores I think.
I can attest to 2manyrats' post, if you let your rats free range they will chew up carpets and things. My first rat never used to be a chewer, but all of a sudden lately she's tearing the faces off my plushie toys and ripping up my carpet. I have to clear my entire room tomorrow (it does need a clean, but the fact that I can't let her wander my room because of the stuff everywhere is an incentive to get it done), so she'll stop tearing apart my books and papers. I'm just hoping she doesn't get a taste for wires.
Like 2manyrats said, they hoard anything that interests them. Nothing is really safe. Doris takes my house key, my nasal medication, papers, anything she can get. It's worth it to keep small things away from their reach. Trying to find the latest nest to get your house key when the bus is coming in five minutes is extremely frustrating. I keep important small things in a jar where she can't get them.
So:
Wrap or tie up electrical wires
Put anything small and meaningful in jars or on a shelf they can't get to
Keep all important papers away from them
Prepare for ripped up carpets
Put stuffed toys out of reach
Make sure they can't get out of the room
Cover open water sources (My Doris used to drink out of my fish tank until I started keeping a glass of water on the desk for her)
I'm sure there's more things I'm not thinking of. Oh, some people also like to put a "rat blanket" over the bed/couch/whatever, as rats tend to constantly dribble.