There is an important reason WHY latin dances commence in a specific direction.. The genre is based upon " call and response ". This is the basis from which the original formats were based. Looking back to origins, the format was/is religious based ( Santeria ) .

In the classic form of Rumba ( NOT the BR one )the lady ( both are dancing solo ) invites the man to approach her, swirling her skirt.. hence the "call and response ".

This would dictate that, the latin dances should commence forward, but, there's another important component to recognise..and that's, the direction of the 2nd beat in the 1st bar, again, giving us direction to commence ( forward ).

All chain schools , taught most of their latin dances, commencing back . ( Square Rumba/Bolero being the exception,) and F.A. side ways for Rumba . The prep. was the intro. to the dance..

Actually, there is possibly a reason why the backwards commence came into vogue.

" Bolero ".. is the dance ( and Danzon ), that was long established, before the current formats were invented.
Its pretty certain that Bolero, stepwise, along with Lindy,did influence many of the foundation steps in Rumba, Mambo and Cha cha ( along with Guajiras influences on Cha cha ).

I can only guess, but in dances that break on 2 (Cha-Cha, Mambo) the 1 beat is a prep step--implied in the case of Mambo. Following the "man's left foot moves first" (on 1) rule, the first break is a back rock.

Intern. style Rumba can prep. on 1 (R foot ) and then breaks forward on 2 .

The ways of commencing specific dances, are often rooted in precedent, and again, changed to suit circumstance. There is a good argument, in each case .
B/room simplified pretty much all, by commencing Forward .