Sushi rice vinegar proportions
I did an online search for the first 10 sushi rice recipes with distinct ingredient proportions and came up with the following ratio (by weight):


  1:0.08:0.16:0.02 (Raw Rice:Sugar:Vinegar:Salt)


With cooked rice,


  1:0.03:0.06:0.01 (Cooked Rice:Sugar:Vinegar:Salt)


Or, if you prefer:


  For 1300g / ~6 cups Sushi Rice
  
  400g or 2 cups of raw sushi rice (about 1200g or 6 cups of cooked rice)
  32g or 3 tbsp white granulated
  sugar 63g or 4 tbsp vinegar 7g or 1tsp salt


I've not tried these proportions yet but I usually find that a simple average of 10 to 20 distinct recipe proportions gives me a good starting point for experimentation. At the very least averaging helps avoid nasty surprises like the saltylicious recipe of Alton Brown I linked to in my comment above.

Another answer here suggests a Salt:Sugar:Vinegar ratio of 1:2:2, whereas my figures suggest something closer to 1:5:9 if we are talking weight ratios (the other answer doesn't specify).

You asked whether salt would help give the rice a bitter sweet taste. In my experience sushi rice has no obvious bitter taste, but it does have a sweet-sour flavor that the vinegar and sugar lends. Salt helps other flavors play nicely with each other and at the same time adds a delicate flavor bass note of its own.

In my opinion you are unlikely to find a favorite recipe by adjusting the proportions alone. There are preparation techniques to play with, varieties of rice and types of vinegar. Many sushi rice recipes feature mirin, sake, kombu or dashi in the list of ingredients. Experimenting with any of these things will likely necessitate altering the ingredient proportions from what I give here.