Here we go again--
It depends largely on where you get your info.
Initially (back in the 40s-50s and still today in chain schools ) the man was taught to commence side left-- break back on 2-- this is named the " call and response " theory ( some say fwd on "2" is more correct ) and follows the clave rhythm, which is more accented on "2" of the 2nd bar in the 2 bar sequence( on 6) .
Commencing fwd is also very common ( on 1,2 and or 3 )the reasons given all have some validity .
If you are really interested in indepth origins, there are numerous sites that will supply all that info.( Salsa Forums ) ( I could do this, but it takes an hr to type )
Bottom line-- choose what makes you comfortable-- and try the different styles