confused about using propagator

Hi folks, Ive grown quite a few seedlings using an unheated propagator but decided to buy a heat mat to move things along a bit. Ive got 5 desert rose seedlings, each of them in a little coir nest thingymabobs and ive placed them inside a sandwich box on top of the heat mat. Now this is where i get confused (even with the unheated prop). Apparently once seeds have germinated they should be removed. If for instance, one germinates before the others. should i remove the lid from the box, or should i wait for the lot to germinate before taking the lid off? and do they have to be removed from the heat mat as soon as they germinate?
I hope this all makes sense to someone, because im beginning to think it doesnt make sense to meself
Many tks in advice

 

Thanks for that JWK. That clarifies things. I shall remove any little fellas that come up early and move them to the windowsill. Many thanks