My husband and I are planning a 15-day trip to Italy in May 2019 - flying in to Milan and out of Rome.

Currently, I'm thinking of hitting up the following places:
Milan - 1-2 days
Emilia Romagna - 5 days to explore Parma, Modena and Bologna
Tuscany - 5-7 days to explore Florence and the Tuscan countryside (and possibly Siena/Pisa)
Rome - 1 day

We love food and wine, hence the visits to Emilia Romagna and Tuscany. As we can't eat and drink the whole day (or perhaps we can), we want to squeeze in some art and architecture (hence Florence) into our trip.

The objective is to minimise the number of times we change accommodations, and so, are looking for recommendations for places we should base ourselves out of for those 15 days. We will be renting a car after we leave Milan. Appreciate any tips from this well-travelled group. Salut!

People usually visit Parma, Modena and Bologna (as well as Cremona, Mantova, Ferrara, Ravenna, Classe and Faenza nearby) for art and architecture.
I would choose one base in Emilia-Romagna and another one around Florence (may be a third one around Chiusi). It's up to you whether you prefer a historical city center (longer driving times than from a hotel along the highway) or a place with easy access to the autostrade.
I suppose the night in Rome will be a technical stop, as a somewhat decent visit of Rome would need at least 3 full days.

Unless you have planned something specific to do IN Milan (and for many there is plenty although you don't hear too much about anything around here besides "The Last Supper" and the duomo, unfortunately) I would not stay there for even one night. You can very easily get a train to places such as Bologna which might make a good base for you.

Thanks for the ideas, @neckervd!

Is Chiusi pretty accessible to the hillside towns in Tuscany? We're planning to hit 1 town each day, with visits to wineries as well. What would the driving conditions be like? And you're right, Rome is just a technical stop as we're flying out of that city.

Depends on what you mean by the hillside towns, but yes, Chiusi is close by many hillside towns, including lots of small out-of-the-way old Roman spa towns up in the hills that few tourists visit. And Chiusi has an axceptional Etruscan museum and underground city. It's a very enchanting place.

Driving conditions are fine, nothing to be worried about. But I always tend to stay off the autostrade and drive on the smaller roads.

Chiusi is close to Montepulciano which could be a good base for Tuscan hill towns and that is one of best.

Just a logistical thing, but since you will have a car and Rome is not a priority on this trip, instead of a laborious drop-off, train in and out of Rome or airport hotel, you might consider driving down the coast so you can drop the car at the airport. Provided your flight is not terribly early, there are some nice spots in southern Tuscany, northern Lazio that would provide a relaxing last night.