Bowl shape pond or not?

[xtreem3d]

hey guys,
i'm a fert, lawn ,snow guy not a pond guy. Doing a round fountain in a 2.5 ft tall stone circle in the middle of my driveway. i used my excavator to dig the majority and now it has a round bowl shape. I seem to remember someone telling me to shave the walls straight up and down ..not to leave them sloping like a bowl. Can anyone help and explain the pro's/cons? if it matters it's about 25 feet diameter and about 24-30" deep. I just want it to be a round fountain , with some type of fountain head and about 3 to 4 underwater lights and it will have flowers planted around the 3 foot border (stone walls is about 40 feet total diameter)
thanks,
steve