We traveled through Rome and went sightseeing on our own. Most places in Rome can be seen on your own happily with the RS audio tours or with a good knowledge of the places by reading about them ahead of time. There's some very good documentaries on the Colosseum and Pantheon for free on YouTube or purchased inexpensively from Amazon etc.
We did pay for a tour of the Colosseum from one of the people standing around outside the Colosseum to book tours and it was a mistake... the guide knew less about the building than we did and couldn't answer some basic questions!
In retrospect If I could do it again the one place I felt like we needed a good guide was the Roman Forum...its amazing but also very battered/in pieces so its difficult to understand unless you have a guide. We snuck up behind a group with a different guide who was very good and really brought the place to life we really wished we'd been a part of his group!
So my advice: most places can be explored quite well on your own if you do your homework. If you decide to use a guide for specific locations research and research some more to find a guide who is a good fit. Maybe give them a call or an e-mail and see how helpful/accommodating they are ahead of time.