I agree with the poster above me about multi specing.
i have 2 lvl 45rogues(1 alliance and 1 horde) and for both of them there spec is this
http://www.wowhead.com/?talent=whecotZG0VbZh
its 21/12/3 the 3 in MoD is there cuz i like to solo alot, but when i get to the point where i dont want to/cant solo ill respec to this build at 70
http://www.wowhead.com/?talent=whecox0oZGhVb0E00bGRZV
personaly i like to maximize my crit% and DPS output(but then again who doesnt right?) at lvl 45 my crit % on both my rogues is about 18% wich is pretty good for that lvl im told.
in my opinnion the 1st two tallents should go into improved sinister strike, u use that ability more than anything else imo.
then the next 5 in malice, u can never have too much +crit%
then after that its all a matter style, if u wanna be stealthy and go from behind, do a combination of assasination and sublety, if u wanna face the mob head on, go combat.
again its all a matter of playstyle, on my 1st rogue i respec to like 50G b4 i got to the 1 i liked lol, just try them out and go with whatever works best for u