I've been paying close attention to this issue. For me, it depends on the intensity of the workout. All my workouts are running.
As CatLady06 said, aerobic tends to lowers it. My BG keeps dropping as I increase my speed....up to when I get to Gray Zone 3. That's a little faster than threshold or what some people call lactate pace. At that point my BG skyrockets!
As for why you rise AFTER exercise, a lot of this depends on the type of exercise. But generally you will be using muscle glycogen to fuel your exercise. The rise when you are finished is your liver trying to replace muscle glycogen. This is normal.
Experiment with post-workout insulin dosing to see how much you need. I always take a big dose immediately AFTER a workout, and then take in simple carbs to quickly restore what I burnt.
Check your BG during your workout to figure out the dosage timing and amounts you need. I experiment on myself everyday!
Please let me know if you want to discuss specifics and type of workout.