Rock Car Port - Do I need to use fill dirt

Smooth it out. Otherwise, you'll have to fill some areas deeper with rocks than others, and it won't compact or wear evenly.

I would add this

Use a road base, not a gravel. It includes a much wider range of particle sizes and will compact and lock together much better than gravel.

I would use 610 limestone, will cost a little more but will be almost as hard as concrete

Ill take this a step further and say start with knowing the height of the finished surface and work backwards. If gravel is what you want then know how thick its going to be. Then, you may need to excavate some topsoil to get down to a solid base of soil. Then, get a plate compactor and make sure the soil is graded properly ( for drainage) and compact the soil well. Then, bring in the 6-10 limestone in three separate lifts. Example, if you want a four inches gravel then you will need six inches of 6-10 limestone in three separate two inch lifts. You bring in two inches and compact it well. Bring in two more and compact it. Then the final two and compact it. You don’t want to dump all six inches thinking you will compact it down to four. Wont happen and the base will fail. 6-10 limestone is very predictable. Don’t use regular limestone.