Is this a good idea?

I'm thinking in opening my software consulting company, do you guys think it's a good idea to do the first few for free (only small ones of course)?

Would you mow this potential buyer's lawn for free to earn their business? Incentives are fine, but they need to be tied to a sale, not just an act of good will.

Doing free work is ok as long as you're doing it for a charitable cause. I wouldn't do free work for someone who aught to be paying for it though. 99.999% of the time you won't get anything out of such an arrangement, not even recommendations.

Any legitimate buyer of your services will be willing to pay. You can always write it up such that the risk is all yours, so that they have to be completely satisfied before you get paid, and their cost is fixed. But if you take such an approach, make sure you embed copyright notice that prevents unauthorized usage in your code and don't transfer the rights until you're paid. That way, you have legal recourse if they refuse to pay you but still use the code (which happens a lot).

Some freelancers also include auto-disable features so that they can't use your code for very long without paying you.