Coil winding sense in dual voltage power supply
Since the output of the rectifier is full-wave rectified it cannot make any difference which way the transformer coils are wired.



Figure 1. Full-wave rectified signal will be the same for either polarity of AC.

You can save one rectifier by using a "centre-tapped" arrangement.



Figure 2. The centre-tapped arrangement uses one bridge rectifier.

This arrangement is also a little more efficient as the current flows through only one diode on its way from the transformer outer taps back to the centre-tap resulting in a voltage loss of only 0.7 V compared with 1.4 V for the arrangement shown in the question.