With Universal you don't really have to be concerned with that IMO. RP is the closest to IOA and is where we normally stay. But say if you were interested in going all out, super plush, the Portifino is the nicest resort on property. It is a long walk, but we never walk into the parks from Portifino and always use the boat. Its a very nice, fairly quick relaxing ride, and very cool way to get in and out of the resorts to the park areas. When you depart off the boat in the City walk area, its the same port you would come to if you came from RP, (the boats all end up at the same place.)
Now if for some reason you absolutely have to walk, RP is much closer to IOA for the walk. But we only walked about 10% of the time because the boats are so much fun to use IMO.