There is a custom to fill the havdalah cup all the way until it is overflowing as that is considered a siman brocho, a sign of blessing. It is not an issue of bal tashchis, being wasteful, or bizui ochlin, treating food disrespectfully, as it is only a small amount that is being spilled out, especially today when virtually everyone has a plate underneath the cup and the spilled wine is not wasted anyway. Some authorities suggest that this should also be done with any kos shel brocho, and indeed the Rebbe as well as some Sefardi poskim, suggested it should be done every single time we make kiddush. In this way, the overflowing of the wine which symbolizes an overflowing of brochos will bring a siman brocho not only in the beginning of the week on motzei shabbos, but every time we make kiddush and do other mitzvos which require a kos of wine, such as a bris, chuppah, or sheva brochos.