That rule is really for Pop music and is not really applicable for classical music.
1st beat and also the 3rd beat, but this is not engraved in stone, it all depends on the piece.
If you are playing solo, you can also accent the 2nd and fourth beat with the left hand, as this is the beats a drummer would accentuate, especially the fourth to announce the next mesure. That is why the snair is mainly used on these two beats. Again, this is not engraved in stone, but it works for almost all pop music.