The halacha can be found in ???? ??.
According to some, folding on the creases is a problem. This seems to be the halacha in the Shulchan Aruch. The Aruch Hashulchan however says ?????? ???? and he explains that our folding is not the kind of folding that is assur.
R’ Ovadia in Yechaveh Da’as 2:40 says that one should be machmir l’chatchila not to fold on the creases. He adds that one should not protest those who fold normally because they have what to rely on.
Personally, I know a few rabbanim who are meikil l’chatchila.