If a domain name will have more than 2 words, I dash it. 2 words get no dash. That is only done if I can register it one way and I need to make a choice.
I will always try to register the domain name both with and without and use re-directs for the second one. When you're telling someone to go to your domain, it's much easier to tell and to remember without the dashes. If I have a domain that I'm printing on business cards, t-shirts and such, I like the readability of the dashes.
Bottom line is register both if you can.