The natural appearance of a no-mortar wall is attractive and complements natural landscaping. Keep in mind though, depending upon the type of patio you build, water from the ground will flow between the stones and come out on the patio. If the patio is made of pavers, this isn't a problem. On the other hand, if it is concrete, the water and mud may collect there.
A sitting wall is actually just a retaining wall, so its first purpose is to retain the soil. Instead of building a true dry-stacked sitting wall, consider building one with mortar between the back half of the stones. This holds them together better and the exposed front still looks like a dry-stacked wall.
It would be wise to make some provisions for drainage through the wall. Even with a low retaining wall, the weight of the water behind it can create quite a powerful force to move it. Also, constant dampness behind the wall isn't good for the life of the stone or for your comfort. You want to smell the sweet fragrances from your flowers, not musty ground.
The sitting wall can be placed over a concrete or a crushed gravel base. If you are going to finish the top of the wall with some type of solid masonry cap, either type of base works well. Gravel is actually easier to install and provides some drainage under the wall.
If you plan to just use some of the larger flat stones for the top of the wall, a concrete base is best. Throughout the seasons, the ground at the base may move up and down. With a concrete base, the entire wall should move together, keeping an uncapped top more even.
Dig a trench below the ground level for the concrete base. Locate the trench several inches back from the edge of the patio. The base should be several inches thick and contain steel reinforcing rods. These rods will help it remain solid and move as one unit.
While the concrete base is set up, sort through your stones and arrange them according to size. Plan out your first row, which will rest on the concrete base. The width of the stones is more important than the thickness for this first course. It will be set in a mortar bed, which can accommodate a difference in thickness.
For subsequent courses, the thickness is more important. As you stack them, make sure to keep the mortar on just the back half so it does not show. When it is set, fill in behind it with filter fabric, gravel, and a four-inch drainage pipe. Level the soil in your garden up to the back of the wall and enjoy the view.