A coaster can be made of paper, wood etc and is not always round. Coasters can also be used with cups, glasses mugs etc. A saucer is normally made out ceramic or glass, is almost always round and is part of a matched set of a cup and saucer. A saucer normally fits to its specific cup and is usually part of a crockery set. I wouldn't necessarily associate 'impending' with any negative connotation. I use it quite a lot actually. I suppose one might use it in more formal settings.