Any advantage in using coil spring isolators on the bottom as well as top of springs for road noise?
There is no real advantage.
The isolators are used simply to keep some of the road noise out of the suspension. This is some, not all. You probably won’t notice a difference using them on both ends, vs. just one end.
A little tip – if you are changing springs, or just the rubber isolators, use urethane for they will outlast the rubber style many times over.