Is a City Tour in San Diego a must?

Sadly.....no.  In the age of Google Earth and online videos, you can see a city more effectively than on a bus.   Plus, San Diego is so large and diverse, that you are really best served by deciding what interests you. If you like history, a historical bus tour through Old Town San Diego and the Gaslamp Quarter might interest you.   If you like the beaches, then a coastal tour (or just a ride on the Coaster train) might be the best option.   But some of the best places in San Diego are not served by tours:  San Elijo Lagoon, Torrey Pines State Park, Imperial Beach/Border Fields Park.   My recommendation is if you want to see downtown and Balboa Park, use GoCars tours.   It's a blast!

Whenever I visit a new city I always try to take a Hop On Hop Off tour or something like that just so I know my options. San Diego is a beautiful city with a lot to choose from but if you don't know it well, you might miss some of the highlights. My suggestions: Coronado (California), Balboa Park, the San Diego Zoo, La Jolla, Del Mar and Torrey Pines.

The Downtown San Diego part of San Diego is interesting but not close to the best part of San Diego. I would recommend hiking down the cliffs at Torrey Pines or touring Old Town San Diego. Also, Del Mar has breathtaking scenery and great shopping. 

It's not a must, but it's a great way to get an overview of the city and surrounding areas. Then go back to the sites you want to explore personally in greater depth by car. Pick up Old Town Trolley Tours in Old Town San Diego, an important (and fun) historical site itself and decide which tour option makes sense for you.  They are hop on/off so you can spend as much or as little time at each stop as you want.