The SQL Server evaluation's download page mentions that the evaluation license is good for 180 days, and specifically advertises it as a tool you can use for mission-critical applications. This tells me MS is fine with your using it for development work.
To answer a question with more questions:
How long does this project run?
What phase of the effort are you in now?
Is this an internal/proof-of-concept project, or something that your customer(s) will be using for a long time?
If you are going to need to use SQL Server for Operations & Maintenance support months past the initial evaluation period, you ought to get a license for the full version of it. And also consider what your customers are using so that you can reproduce any bugs that come back from them.
I don't think it's ethical to continually renew evaluation licenses to have a longer evaluation period. Companies call them "evaluations" as a try-before-you-buy, not a keep-trying-without-buying.