That depends entirely on the size of your project.
If the entire comic is going to be just a handful of chapters - then I'd say go ahead and finish the script.
But if you, like me, are going for a HUGE project with lots and lots of pages, that's going to take a very long time to finish even if you do start drawing it right now? Then I'd advice you to start drawing right now, and finish the script as you go. Unless you have a very definite idea of what you want to do, it's entirely possible that you might change your mind about stuff along the way, and it's much easier to change things when you haven't set it all in stone.
In the end, do whatever you feel is right.