In Aikido the old phrase "He who hesitates is lost" is king. In my dojo we practice what Bill Sosa Sensei refers to a shodoseisu (controlling the first move). This doesn't mean that we will always use atemi or geri. Often it will involve control of ma ai (distance). Whether or not this is done by moving towards the person, off the line, or away from the person is dependent on the situation. I would disagree that Aikido is "reacting" to a situation. This implies unpreparedness. I prefer to respond to the situation. This implies thought before action. So, I guess I put alot of emphasis on being aware of my surroundings, and what situation I am putting myself into. This is not to say there is no reflex action in Aikido. I believe the basics should be reflexive, but what you do after that should be cognitive. (i.e. you avoid a punch and control someone with ikkyo--but do you reflexively throw them in front of a bus? No, you avoid hurting or killing them and circle them gently to the ground without harm).
To paraphrase Rickson Gracie "I am afraid of everything...that's why I've been around so long. A little paranoia is a good thing."
So stay alert and be as prepared as possible.