Hi Wilko,
sorry for my late reply. I hope you are also doing well!
I think both options are reasonable. I often used this function to visualize transfer flows between different buses, therefore I had to sum up negative and positive values. When you want to see the mean loading of the line e.g. to see how often it is used, the absolute values have to be summed up. But it is definitely wrong to do it differently for active and reactive power.
I would suggest to have both options; 'line_loading' (with .abs()) and 'line_flows' (without abs()). The arrows on the lines would be used only in the 'line_flows' plot.