As Jock says.
I also was taught to tighten in the orientation shown. Final tightening shoudl be done this way, flip it over for initial slackening. When moving freely, spanner can be used either way for access.
It shouldn't matter really, as both jaws should be strong enough to resist splaying. I guess the rule is very old, from when spanners were possible weaker, although they would have been rather fatter years ago.
If using an adjustable spanner, it might matter a lot. An adjustable spnner ini the orientation shown should only be tightening. Pressure on the adjustable jaw can lead to slip of breakage, of the spanner.