[hcrisp]

I heard a maxim that declares: "a sharp discontinuity in the frequency domain equals ringing in the time domain, and a sharp discontintuity in the time domain equals ringing in the frequency domain".

In other words it will cause ringing (oscillations, Gibbs phenomenon) in the time domain (for signals, or spatial domain for images). If instead you want a smoother result, you will need to use oscillation in the FFT domain when zeroing out bins. As that is hard to do perfectly, it's easier to specify a window function (or a classically derived FIR or IIR filter kernel) and convolve it with your input signal / image. It is also more efficient to do online when the data is streaming.