I fitted some fitted wardrobes in alcoves for a customer last summer, one of them had it's side and back to a solid external wall (about 50mm gap between though). The wardrobes were a very traditional style with a large cornice around, I had planned on leaving a 6" gap above them to the ceiling but the customers wife wanted the fully fitted look so the gap got filled and the infill painted the same colour as the walls.
Just before last Christmas I got a call saying they were having an issue with mould forming in the wardrobe, I think it didn't help that it was a guest bedroom so the wardrobes were stuffed full of spare clothes but never got opened.
I fitted some grilles/vents in the skirting at the bottom and drilled some vent holes just above the cornice which you couldn't see unless you were up a ladder, this got some air flow to the cavity behind the wardrobe. I also suggested they open the doors on the wardrobes now and again.
It seemed to solve the problem and I have done loads more work for them this year.