How does reverse leakage current of a Schottky diode come into play with a SMPS?
Schottky diodes are great for SMPS applications because they have no reverse recovery. (Some Schottkys at higher voltage have guard ring structures that can cause some reverse recovery-like behavior.)  However, they do exhibit higher leakage current than conventional diodes, especially as the applied voltage increases.

The only downside of the extra leakage is a loss in efficiency.  If the supply is to operate at high temperatures, and the input voltage is relatively high for a Schottky diode the leakage loss can be significant enough to warrant investigation.

In general, people use Schottkys in switchers without even worrying about it.