In terms of current draw, there is no real reason to overspec a SMPS. Once it's overcome the initial start-up surge and charged up it's various internal capacitors, and the stepper driver capacitors, the average current draw is minimal at idle/slow speeds. At idle, with say a 5V 2A stepper motor, it should be seeing a combined total of 2A at 5V through either or both of it's windings, so 10W. Now say you've got a 2A 50V SMPS, that's a 100W source. The stepper driver (which is essentially a fancy SMPS), although switching currents might peak at 2A, the various capacitors will smooth that somewhat before it reaches the SMPS circuitry.
Even at higher speed/load where you happen to be in the optimum point to be applying 50V to get the full 2A through the motors, due to the switching nature of the stepper drivers you're not likely to draw the full 100W capacity. Plus once you get beyond the optimum voltage/current point, back emf will limit the amount of current the available voltage can push through the stepper motor windings, and power consumption will actually reduce. This is why it's recommend for a typical 3 axis machine, you only need a supply capable of delivering 2/3 the maximum combined current.

The real big problem with SMPS's is there inability to handle the regenerated energy from stepper motors/drivers as axes are decelerated. During deceleration all that kinetic energy gets dumped back into the power supply, which can cause it to go overvoltage. SMPS rarely have the ability to handle that, with the better ones simply shutting down (often until they get power cycled), and the cheaper ones releasing their magic smoke. Bigger SMPS will normally have bigger capacitors so they'll absorb a bit more energy before going over voltage.
You can get around this by installing a reverse energy dump circuit which will dump any regenerated energy to a big resistor rather than into the supply, but for the cost, you should be able to build a suitable linear supply that will avoid all these problems, be far simpler, and have less bits to potentially go wrong.