I strongly recommend you not work for free. When starting out you can certainly work for cheap, even dirt cheap. But working for free gives the wrong impression to developers that an audio guy's time, talent, effort and enegry (and even the audio itself) is worthless. I've also seen, personally, certain developers hop from free audio guy to free audio guy.
The best way to be seen as professional in this industry is to act professional. This means expecting to have a written contract and terms which you can agree and commit to. It means meeting deadlines and being open to feedback/criticisms of your client and making the needed changes. Being on time (if not early) to meetings, etc. All of the things we'd all look for in a solid, good co-worker.
It always makes me wonder (if that's even the right word) when folks advocate doing everything else in a professional manner (i.e. how you're networking, your website, having a great demo reel, etc) but then when it comes to money there's a disconnect and that working for free is somehow professional or appropriate. It's not. It harms the industry and others, perhaps more than most are even aware. If it's a purely hobby project which will never be sold and no one's getting paid - that's completely different. But if it has commercial aims and some folks are getting paid, then everyone on the project should get paid as well.
After all, all of your hardware and software wasn't free (or it shouldn't be.. lol) and if you attended any classes or got a degree in this... that wasn't free either. Even if you got a full scholarship, someone paid for your education. Even if it was just you learning at home... your time and effort in learning how to do this well is worth something!
So my advice, work for something. Be it money or an exchange of services. Explain to your clients that you're just starting out and because of that you're super discounting your content while you build up credentials. Not all credits are equal. Don't give away your time and craft just for a credit which, in reality, a small number of people might know and have played. I've been long winded but in the end... I think this is the best way to build up a solid business model and rep in the industry. But if you're dead set on working for free at the start - at the very least keep all of the rights to the content. Never give away exclusive rights to content for free. This way you can at least re-use and resell it later down the road.