It is generally the case that instances are run in IID order, but not always - using families will mean the instance order is a mish mash of different object types, and I can't think off the top of my head how that is handled. Still, I wouldn't rely on this behavior at all, because it could change in future engine updates.
To be honest, it's a very unusual question. Why would the order the instances are run in be important? If you have two separate events, as far as each instance is concerned they are affected in the order of the events, even if the engine ran the instances in a random order. It's actually quite difficult to make an event where the order instances are run affects anything. Why are you asking and are you sure your events have this requirement? You might be mistaken, and in fact the order instances are run in does not affect your events at all, even if you think it does.