Back when dwell angle meant anything (when we all had points in our ignition systems), 30% dwell meant that the points were closed and the coil conducting for 30% of the time. As RPM went up and down, the dwell stayed constant. Which meant the coil conducted for a longer period of time at low RPM than at high.
I've got a project involving building a new electronic ignition module for a vehicle, and I've never looked into how electronic ignitions handle the dwell issue.
Once a coil has 'charged up' so to speak (the DC current has risen to a steady state value given the coil's primary inductance), allowing it to conduct for a longer period of time until the points open buys you nothing. The stored energy in the coil is the same.
Back in the days of points, it made more sense just to let the closed time track up and down inverse to the RPM. A simple cam will suffice. But now, its possible (and actually easier) to build a one-shot circuit that breaks the coil primary for a fixed period of time regardless of RPM. Of course, that lets the dwell angle go all over the place. But as long as the coil conducts for the requisite minimum amount of time, there should be no problem. In fact, such a system is a back door way of implementing a speed governor. Once the RPM goes so high that the fixed 'open' time exceeds the time needed between sparks, the engine effectively shuts down (the crank angle sensor keeps resetting the one-shot before it can time out and close the circuit).
So, how do modern electronic systems work? Still constant dwell angle? Constant time? Granted, the ECU controlled ignition could easily be implement some sort of active coil current control. But that's going a bit overboard for my needs.