For ground operation there are three possible sources of ice to be shed. One is ice adhering to the back side of the fan blades (most common to freezing fog) The other is ice adhering to the nose spinner on the very outside front of the engine (where the black and white appearance keeps changing as you taxi at different power settings) and the third is the core inlet vanes behind the fan blades that can only be seen by a few passengers on one side looking through the fan blades from their window line.
The idea behind shedding the ice on the rotating components (the first two) is to get it up to a high enough centrifugal load to shed all the ice. Since the ice never sheds evenly some vibration may be experienced as you spool up.
For the later source of ice (on the non rotating inlet vanes to the core behind the fan) you need windage loads to break the ice loose (there is just not enough heat to melt it loose). most manufacturers recommend 60% N1 to provide this windage (read the manual and check). Again the idea is to shed this ice before it becomes thick enough to cause downstream compressor damage. If you don't perform this operation often enough then its sure to shed at takeoff power and result in downsteam damage to the blades. The greatest problem is with internal blade downstream damage causing the engine to surge shortly after rotation so be prepared and follow your manuals faithfully.