My 2 cents...
I have had the same dishwasher for almost 13 years. A MOL Whirlpool. I have never put detergent in the pre-wash cup. It is so short, it seems a waste to me. It looks almost brand new inside. Even if there water is greasy and disgusting during the pre-wash, there should be plenty of detergent, and hotter water during the main wash to get rid of any residue. I never pre-rinse, and I barely shake off the food before I load it.
I also have mechanically softened water. Filling the main wash detergent cup more than 1/3 full causes excess suds and will etch my glasses. Tabs are wayyyy too much detergent for me, even with really heavy soils. While admittedly, the loss of phosphates has affected detergent ability, I rarely notice the change due to soft water. I do notice that an occasional pan will not come clean these days, where it would have before, and I know that is due to the detergent. I would not recommend tabs to anyone unless they had pretty hard water, and a machine that performs better with them (like Bosch or Miele). I think the dosing is too high, otherwise. To add to that, they are much more expensive per use, and you have no control over how much you are dosing.
Just my thoughts....
Joel