There's some conflicting information about this but it seems that it is generally not advicable to mix beta blockers with amphetamines (and many other stimulants). It can be dangerous for your heart, in the worst case causing damage to the heart. I'm not trying to sound like an alarmist here, but think of it this simple way: the stimulants raise your hear beat and propranolol lowers it. One drags your heart rate up, while the other drags it down, so your heart becomes a "fighting ground" between the two. This is not good for your heart. In fact, instead of removing unwanted side effects you may get a an irregular heartbeat and cardiac dysrhythmia. It's true beta blockers reduce hypertension, but be careful about your heart. Also, the point of taking stimulants is to get...stimulated. Beta blockers are the opposite of this. You won't feel nearly as stimulated if beta blockers keep you heart rate down.
Beta blockers can be good for residual stimulation, when the effects of the stimulants wear off but your heart is still pounding and you maybe can't get sleep. So definately, take propranolol after the others. It should be the last one. And preferrably while the stimulants are starting to wear off. If I take anything stimulating, I always take beta blockers when the drugs main effects are over or when I'm going to sleep. If you want to reduce side effects of the stimulation, take benzos. They're completely safe with amphetamines and removes irritating and nervous stimulation.