A well-tuned, designed, toilet fills the bowl just to the point of overflow when the tank reaches full. Older toilets tended to dump much more water into the bowl, essentially wasting it. If the bowl isn't full when you start the flush, your flush will NOT be as powerful as when the bowl is full to start with. Instead of immediately starting to push the water out, some of it must fill the bowl the rest of the way first, and that affects the overall flushing efficiency. This is why it is important when trying to minimize water use to use the correct parts, properly installed - not all aftermarket parts will achieve good results while simultaneously minimizing water use. You would not notice the bowl was being overfilled, but you may notice it doesn't flush well if it isn't full to start the flush, and definately will notice if the level is very far from the design level.
Now, some toilets used in other parts of the world have nearly no water spot and the waste sits there until you flush. People here tend to not like them, and thus there's little market for them, but they are designed differently than ours.
If you've displaced all or most of the lighter water in the bowl with urine, and wait to flush, adding a little bit can put concentrated urine trickling down the drain. Only when you flush do you cleanse that area. That slow trickle of undiluted urine can eat through pipes, as shown in the picture Terry posted as well as leave crystalline deposits that do not get flushed away. This is real, not theoretical.