I believe the 180 degree roll manouver is to place the shuttle in the correct orientation for orbit. Depending on what apogee and perigee need to be obtained during a shuttle mission is all dependant on the orientation of the shuttle as it climbs- e.g 90 degrees above horizon/ 70 degrees above horizon etc etc etc- it differs. Most shuttles only orbit at relatively low altitudes- 300 km or so and although the altitude of the orbit can be changed/modified while in orbit i think it all comes down to the orientation at liftoff/climbout. And yes the SRB'S do splashdown in the Atlantic to be reused again for a future mission- the fuel tank however does not and burns up on re-entry.