I lag pretty hard in Diplo every game I play, like right now with Castile, closing in 1550ish, I have 10 Admin, 12 Mil, and a whopping 3 Trade. Since the price is down to something like 75 I may actually take 4 soon. I don't need the trade bonus, I dominate Sevile, 100% and even the English Channel (I inherited Burgundy and expanded a bit) with my trade ships. I rake in the money, but way, I have 3 diplo ideas to start, because Influence is awesome, Exploration is necessary for downtime and Diplomacy is awesome for all my HRE Elector vassals, and I am still carrying Aragon until I can get back to 100 Legitimacy and I want Byzantium.
I am often between -1 to -3 dip points, I try not to go more than that, still right now -4 with Styria, Denmark, Poland, Lith allies and Aragon, Palinate, Trier and Cologne as vassals/pus because I will soon be fighting a coalition of France and most of the HRE, including a large Milan and a large Hungary when the King of Lorraine dies.
Dip is just the least valuable if you can dominate your trade nodes without it. On the plus side to get rid of those excess mil points until I get to Admin 14 I have massive manpower heh