Some additional information. While ISFAA can be filled offline, CSS needs to be filled online. Also, sending ISFAA is completely free (you send it as a pdf doucment to your college) but sending CSS does have some charge. (Aks your college if they provide CSS code!) Lastly, it is a lot easier to fill ISFAA than CSS.
It depend on your application deadline and the financial form deadline. Most of the colleges have financial aid form due after application deadline. So it can create your CSS profiel later. Since you have your college board account already, I recommend you to go through CSS as soon as possible because it might be confusing later. I suggest you create profile earlier and be familiarize with it.