If you mean Hawken as in Jake and Sam, the answer is they are both historically correct. A number of J&S Hawken rifles had straight breeches, but some had slant breeches. See Sublette-Beale J & S Hawken article (Topic#241364) under Percussion Rifles.
After Jake died, Sam continued to make both straight and slanted breeches. The classic Sam Hawken plains rifle of the late 1850's such as Carson's in the Masonic Lodge, Santa Fe and Bridger's in the Montana State Historical collection have a slant breech.
It's dangerous to generalize about Hawken rifles because no two are exactly alike, but a straight breech would be appropriate for a J&S Hawken and a slant breech would be appropriate for a Sam Hawken plains rifle. Although maybe less common, the reverse would be true, also.