  We identify the presence of monoatomic steps at the Si/SiGe or Si/SiO$_2$
interface as a dominant source of variations in the dephasing time of Si
quantum dot (QD) spin qubits. First, using atomistc tight-binding calculations
we show that the g-factors and their Stark shifts undergo variations due to
these steps. We compare our theoretical predictions with experiments on QDs at
a Si/SiO$_2$ interface, in which we observe significant differences in Stark
shifts between QDs in two different samples. We also experimentally observe
variations in the $g$-factors of one-electron and three-electron spin qubits
realized in three neighboring QDs on the same sample, at a level consistent
with our calculations. The dephasing times of these qubits also vary, most
likely due to their varying sensitivity to charge noise, resulting from
different interface conditions. More importantly, from our calculations we show
that by employing the anisotropic nature of the spin-orbit interaction (SOI) in
a Si QD, we can minimize and control these variations. Ultimately, we predict
that the dephasing times of the Si QD spin qubits will be anisotropic and can
be improved by at least an order of magnitude, by aligning the external DC
magnetic field towards specific crystal directions.
