How can you use the metric prefixes milli-, centi-, and kilo- to you decide if a metric conversion is reasonable

It all depends on the scale of things.

lets say you have a project where you want to build a model airplane. the units you would use might be centimeters or millimeters, depending on how precise the model is.  But, the wood they sell only comes in meters.

unit conversion would be helpful in determining how much wood you want to buy or how much of the wood you should cut

The correct answer would be "convenience"