If you want the defaults all to 0, just click the graduated filter tool or the adjustment brush and double click any of the labels that are not 0 before you click in the image to make the adjustment. As mentioned, having some adjustment makes it much easier to see what you are doing, even if that's not the adjustment you are trying to make. If you want to add clarity or saturation to a part of an image, it's easier to use an exaggerated setting of exposure to paint the area you want to make the adjustment, then 0 that out and apply the actual adjustment to taste.
I also have the default for my graduated filters at +4. When I create a new filter, I can more easily see the transition area and can position the filter easier. I then double click on Exposure which resets it to 0, then make the adjustments I want. Sometimes I do the same thing for an adjustment brush as a preliminary step then adjusting and often switching on the mask to see where the brush is being applied.
When you select the graduated filter tool, and make any adjustments before you click ink the image, that sets the default. If you click new it returns to those defaults. If you hit escape, deselecting a filter it also resets it and you can just click in the image to add another filter.
However, if you select a graduated filter by clicking the pin, make adjustments to that, and you simply click somewhere else in the image to add another graduated filter, the new filter will start at the settings of the previously selected filter, not at the defaults for a new filter. (which of course doesn't work with an adjustment brush because clicking somewhere just adds to the current brush so you have to deselect it to add another brush).
I did a little
video for my customers about the graduated filter tool if you are interested.
