No! Many dogs needing fosters are URGENT and at risk of being euthanized. Getting approved to foster is not that difficult, but it is a process run by volunteers and therefore it can take a few days or a week. Waiting to apply could be the difference between life and death for an animal.
If you are considering fostering, sign up and get approved NOW. You are never obligated to jump right in; it only helps you be prepared and ready to go when the right fit comes along. Your rescue will work with you as to timelines and helping you find the right match for your home. But get the legwork and paperwork out of the way… get fully approved and then you are ready to go!