When you travel, do you make all the arrangements or do you hire a guide? Do you use a travel service that provides everything or just a guide for certain sites? Are you traveling by boat, air, or car? Are you going to be traveling all over Napa Valley drinking and sampling wines? Will you be hiking in unfamiliar territory?
While there are pros and cons for all of the above, you should decide what you'll be doing and how beneficial a guide would be for your trip.
For instance, if you're going to some location during a time when dozens of tourists visit...you may want to seek out someone ahead of time to let you know the best days and times to visit. That guide may also have special passes or can get you a private tour.
Or, if your time is limited in a city, a guide may be your best bet.
For dive trips, we always hired a guide. We didn't just sign up for a 40 person boat where no special services were offered. You want someone who is familiar with the best dive spots if that's your vacation.
A wine tasting vacation, no matter where, is another time when a guide is a good idea. They may recommend great spots. Better yet, you won't have to worry about drinking and driving.
When visiting places where language is a definite issue, a guide could come in handy when trying to read street signs, order in a restaurant, or get a taxi.