So many things go into planning a festival run that it cuts down on a whole lot of work if it's done beforehand. Even if you decide to go with a Sales Agent, it can help guarantee that your film is getting in somewhere.
Long ago, I had the unfortunate pastime to be working on a film that no one wanted. It had wonderful aesthetics, but sadly, it just happened to fall short of every film festival known to man. I mean no one wanted to program it in his or her festival, and this goes to the top tiers down to the free festivals in countries that no one has ever heard of. The Director wanted to sell the film and get international distribution so the investors could get their money back. Therefore, the movie was made and the Director could not understand why their amazing film that was worked so hard on with massive overtime could not even be screened.
Once you have a script in its first draft, unless you have Hollywood or the Blum brothers backing you, you might want to give some film festivals a thought. It doesn't take a great mind to do some research, just a whole lot of time, and patience. Just taking a look at the website of all the festivals you'd like your film to play in then go through as many past festivals as you can to get an idea of the kinds of films they like to program goes such a long way. It helps with those submission fees.