Question
Do you have to have soil as deep as the sidewalk where they meet along the egdes, or can you plant grass and the grass will make up for the difference?  I hope I am not confusing, thank you for your time.

Answer
Hi Heather;
No you don't, in fact, it will make lawn care easier.
With no soil above the sidewalk, you won't have as much trouble with runoff when you water, less edging. You can fill it up to about 1/2 to 1 inch below the level of the sidewalk, and use the sidewalk for a guide for the outside mower wheels, and it should cut the grass off even with the sidewalk and eliminate the need to edge that side of your lawn.
I wish mine was a little deeper then my sidewalk.
Also, with a little room to spare, if you top dress with anything, it won't make a mess you have to sweep up off the sidewalk.
Highly desirable situation to me.
Bear in mind that as grass roots and flower and shrub roots grow, the soil will expand to make room for them, so in a few years, it should be even with the sidewalk or a tad above.
Lower is better, I think.
Charlotte