Certainly close to all schools, and where children are likely to be crossing. eg my local road is 30mph, but they are about to build 400 homes opposite, which means suddenly hundreds (potentially) of children will be crossing the road in years to come - at the moment there are very few people crossing as no houses on that side. Without lowering the speed limit (and properly controlling it) it would be a dangerous road.
Some high street areas could do with reduction too, just to try to slow down the idiots that don't care. Most people drive sensibly and do about 20mph anyway when driving along a road with pedestrians each side, but not all.
20mph can also make some residential streets safer, especially where kids play out, and quieter too. In my experience, locals are far more likely to complain when people passing through exceed the 20mph limit, than actually object to the lower speed limit.