The word ‘free’ in any keyword is excellent for traffic for obviously not great for conversions. I ill say that if you are able to rank for a ‘free’ term and it’s a natural placement then it cant hurt. I wouldn’t suggest spending a lot or time or resources on this though.
Also I would suggest going for the longer tail terms as well. I have had great experience with those and if you can find some good ones they can really pay off. I believe that the more detailed the search term the more targeted visitors you’ll be receiving. This is definitely a quality over quantity game. (Opposite for the ‘free’ terms)
Good luck man, try using some of the keyword selection tools such as the free one from Adwords for ideas. Also there is a nice tool at SlowSEO.com that I’ve used a lot of times before. Hope this helps!