I don't think you need to put on poly. With the good work you've already done and a high-quality paint, the cabinets will be easy to clean and last a long time. Furniture takes more of a beating and understandable why people might put a coat of poly over paint.

One thing you might do that will help.... after letting the paint cure for about a week. use a good furniture polish on the cabinets. This will help repel any grime or wetness.