Can freelancers use freeware?

I’m actively searching for work-at-home opportunities. Most of my work experience is in clerical and administrative work, and there’s a considerable eLance.com section devoted to people in need of virtual assistants.

But, I don’t have the cash to shell out for Microsoft’s crazy-expensive office suite. And I like to support freeware projects anyway. (GIMP FTW!)

Is it possible to get away with using a software like OpenOffice or Scribus for professional use? Are there any downsides to using freeware? Would I have to disclose the use of freeware instead of Brand X?