I'm developing a SIP parser in C (client only) and i have doubt about, do i need to bind a socket with a specific port (5060) on double nat?. What i'm sure about it's that is really important in the server side but i'm not really sure about the client side
You don't have to use port 5060 on the client side regardless of the NAT type. There is no any disadvantage if you just pickup a random port. The only recommendation is that once you pickup a port, keep that across sessions to help NAT bypassing a bit in some circumstances and to not overflow NAT's with various binding.
Even on the server side you can use any port, but there is a big disadvantage: users need to type also the port part as the server address (yourdomain:port) if you are not using the standard 5060 port.
Think about it like in case of http. On the web server the standard port is 80. However none of the clients (web browsers) are using port 80 on the client side.