We do both depending on what it is we are propagating

following on from above; filling with compost is just for specialist requirements as it requires a lot of follow up attention. It is usually more conveniant to sow/plant in smaller containers placed in the propagator. We add a small pot of water to keep up the humidity.

I tend to use some compost under the seed trays as it distributes the heat and moisture evenly, its quite an old one without a variable thermostat

I just put a thin layer of sand over mine.

Mine has a layer of water absorbent matting that just needs keeping damp. Works a treat.

that sounds a cleaner way Steragram