Pompeii is interesting no doubt but your time can be well spent going to Ostia Antica just outside of Rome, 20 minutes by train. No crowds great remains of this ancient sea port. For my time I would spend it walking in and around the ruins at Ostia than the 3+ hours each way to Pompeii from Rome. The ruins are no better.
Optional from where? It was not an option on the Best of Italy. Loved it. Happy to answer more questions about my tour. You can see pictures at www.lanesworld.net/italy with trip reports.
This was included on my tour.. .. ..if it's an optional-GO! Certainly well worth seeing! Please be careful about the sun, however. My husband didn't use suntain lotion and he burned so badly that he blistered. He almost had to seek medical attention.
On our BOI tour Pompeii was included. The included tour with the local guide covered part of Pompeii, but if you are interested you may want to use the 45 minutes or so of free time to explore parts of Pompeii that the guide skipped. For example the guide did not take us to the amphitheatre, house of vettii, or the brothels with the fresco 'menus' (perhaps because we had quite a few minors in our group). . You will get an excellent map from the local guide and you can use that to find the major sights when you are seeing Pompeii in your free time.
I've been to both Pompeii and Ostia Antica. Pompeii's ruins still have many frescoes, which are nice to see, but Ostia is less crowded, bigger and easier to get to (from Rome.) If you can see both, do, but if forced to choose, I would go with Ostia.
Absolutely loved this place and want to go back to Italy just to walk the streets again. Fabulous with a guide! However, now that I have read about Ostia.. .. ..my interest is further picqued!