Personally I don't think a moderate-thickness black line on all borders would do any harm. I use such a mod in EU4 myself and find it to make the map far more readable. I also prefer it aesthetically over the "coloured" border it normally uses - but that's a matter of preference, of course.
EDIT: added one of my old screenshots for demonstration below:
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I apologize for been off topic but am I the only one amazed at that Byzantium in 1593!!!!!!! ???