If you're trying to stay in ketosis, you shouldn't take carbs, or at least keep it under 50g/day. Typically, you'd want the carbs at night. If you're not experiencing side effects of going very low carb (fast heart rate to the point of anxiety, insomnia, dry eyes/nose, etc.) don't worry about carbs.
If the exercise you're doing is intense and cardio like, or HIIT, or working out several times a week, and you find it very difficult to recover and feel very low energy, like a zombie, then add carbs and forget ketosis. In that case you may need something like 20%-40% of your calories as carbs.
So, if you're working out a lot, what happens is that you deplete muscle glycogen and won't be able to perform as well, if you're running, this is called "Bonking Out." It is possible to adapt to burning fat instead, even for endurance athletes, but it's much, much harder and you'll find your body struggling to provide glycogen for your brain and red blood cells, so you'll dip into gluconeogenesis. Since we don't store protein and that process converts protein to glucose, you'll catabolize muscle tissue to make sugar - not exactly a wonderful thing to do.
So typically, if you're doing intense workouts, or cardio like/endurance like workouts, you'd want enough carbs to be able to perform your sport.
A good book on this is The Paleo Diet for Athletes: http://www.amazon.com/The-Paleo-Diet-Athletes-Nutritional/dp/160961917X/
Here's a quick summary of it: http://home.trainingpeaks.com/blog/article/a-quick-guide-to-the-paleo-diet-for-athletes
Immediately post workout (i.e. within about 2 hours) take 25-50g carbs with 20g protein -- best delivery mechanism is a protein shake and a banana.
You can also take an additional 25g carbs (again bananas are great) 2 hours before the workout.