The arm labelling does make a difference, because if they are labelled incorrectly then the circulating flows past each of the entry arms will be calculated incorrectly.
If modelling a drive-on-the-left roundabout, arms are numbered in a clockwise order. For drive-on-the-right roundabouts, things switch over and arms are numbers in a counter-clockwise order.
In ARCADY 7 and above, arms can be labelled in a more freeform manner. However, the program still needs to know the order of the arms, for the reason given above. In general you don’t need to worry about this because the program will set up the correct default ordering for you. If you happen to insert a new arm after having started a file, use the Junction Diagram to check whether the arm is in the correct position relative to the other arms. If the arm is in the wrong position, you can either type in the correct arm order in ‘Arm Order’ field of the junction, or, alternatively, move the end of the arm to the correct position in the diagram and then click ‘Tools>Set roundabout ordering from diagram’.