Distance in decimal degrees is meaningless. the distance between two degrees of latitude is more or less constant but the distance between two degrees of longitude is roughly 111km at the equator and 0m at the north and south pole. Assuming your lines are short there should not be a lot of north south variation but since the length of the unit of measurement is different in the X and Y axis, length in degrees are basically meaningless.
The length reported by Ipolyline.length will be in the coordinates of the data's coordinate system. This could be meters, feet, nautical miles and even decimal degrees. The thing to remember is that is just the distance in the cartesian coordinate system stored in the database (X,Y coordinate system.) Most coordinate systems depend on a map projection that defines it. Map projections all have distortion, only equidistant projections preserve distance. How accurate is the measurement depends on the projection, how much accuracy you need depends on the application.
For high accuracy over long distances, there is a class called MeasurementToolClass part of the defensesolutions to do that. I have used it back when it was part of military analyst but now it is core functionality.