Depends of course on what you are looking for. Lets take a common example, you decided to join a PPD network and you want to make a landing page, for example named 'fifa 13 generator' if you would have the domain name 'fifa13generator.com' or fifa13generator.tk/fifa13generator.blogspot.com, many people would not trust these free domains thus exit, leaving potential income behind. If only 10 people that were to complete your survey left, this would be around $10. You can easily get a domain at GoDaddy or Namecheap for $0.99 so yes, I would recommend a domain.
As for free hostig, it depends. If you have a large amount of traffic your free host might be too slow or might be cancelled. There is also a large risk of these free hosts cancelling, as they do not have sufficient income. Furthermore if the free hosts would be down %10 of the time this means %10 less traffic, thus %10 less income.