The biggest difference is the lack of a gyros that needs several minutes to align before moving the aircraft (like the one in the A-10C, and that has a very fast modern INS system compared to what was available in the 60s). Since the Viggen was designed to scramble fast to stop an oncoming invasion a traditional INS might not have been optimal for the task. With a skilled pilot i believe i read that the startup time for a Viggen could be as low as 1-2 minutes.
Yes, i believe the pilot will still need to check heading every now and then, runway heading is entered just before rolling on takeoff for example to make sure the computer calculates the traveled route correctly.