There are several things you need to consider. The first is where you plan to use the piece. If your intention is to use it in an entryway or corner as a planter, use a tall stand for a low plant to achieve maximum impact. A tall tree needs a flat stand for stability. If you plan to place the fishbowl or vase on a side table or cabinet, a flat bowl will keep the bowl from being too dominating.
Proportion is very important.
The stand gives the piece a finished look. Without it, it looks as though something is missing. A quality stand enhances the piece. Think of a man dressing to be the best man in a wedding. If he wears a tuxedo with sandals instead of dress shoes, he will look incomplete even with the fanciest tuxedo.