I think a specific answer to your question depends on a few factors: how much research do you do on your own before traveling, how good are resources already available at the sight (like audioguides, signage, etc.), how much interest you have in a particular place. I'm a fan of guides in specific situations where I really want in-depth knowledge.
I have been to Spain a few times and am currently coordinating a two-family return trip in December. It will be the first time in Spain for half the group, and most of the rest will not have been to all of our destinations. I plan to use tour guides in a few places, primarily because there will be 6 teens/young-20's in our group who I know will NEVER do any advance research on their own, but I also know that they are very interested in background information and historical context, and they just won't get this without someone speaking directly to them at the time they are viewing the object of interest!
We are traveling to Toledo, Granada, Cadiz, Cordoba, and Madrid (one night only in Madrid, no art museums). I'm planning to line up tour guides for the following:
-- Toledo -- about 2 hours for a walking tour of Toledo and to give historical context of the city itself. Specific sites in Toledo that I have been to have excellent available written or audioguides (especially the cathedral), so I don't feel the need for a "person" guide for those. Also, if you have Rick's Spain guidebook, there are some very good, thorough "tours" of specific sites, so be sure to use that resource if you have it (I like the electronic version; I can pull it up on my phone while I'm out and about).
-- Granada -- tour guide for the Alhambra. I've done this before, and I find it just makes the visit so much more meaningful to have an experienced, knowledgeable person who can bring the place to life as it was over the centuries. Three years ago, I used Margarita Ortega Ortiz de Landazuri, recommended in Rick's guidebook. Her partner Maria did our tour, and it was so good.... really, really added to our time in this wonderful treasure. We had her for nearly 4 hours (supposed to be 3, but we had tons of questions, and she was very happy to stay with us until we had seen and talked about everything we were interested in) for 130 euro. A fantastic investment. I will contact her again for our trip this year.
-- Cordoba -- like Toledo, I plan on finding a guide to give us a roughly 2-hour walking introduction to give us historical context for the city, which was so important long ago, but difficult to understand now unless you either do the research beforehand or have someone guide you through.
I will probably also check into some type of food or beverage tour in one of our destinations. We've done this is the past and find it a great way to get some cultural background (food is important!) as well as explore some bars/food/drinks that we might otherwise not have tried on our own.
We are not planning to go to Sevilla on this trip, unless we go for the day from Cadiz, but I will note that the audioguide for the cathedral is terrible; last time I went, I wound up reading aloud from Rick's guidebook so that my family could make some sense out of what is an overwhelming number of notable items packed into an absolutely enormous building. You might consider some type of tour so that you can appreciate the importance of all you will see.
Also, for Madrid, if you are truly an art fan, then think about a guide in the big museums like the Prado and Reina Sofia. I find that art is so much more meaningful when I understand the background and history surrounding the object, and there is SO much to see in Madrid that a good guide might really enhance your experience.
So, that's my 2 cents. I hope you have a wonderful trip!