Thin set or mastic?
Because the pattern of pebbles is variable, I like to dry fit as much as possible before I begin the setting process. This is a time consuming task and it is very difficult to rush.
A good majority of pebble installs are on shower floors. Like a more traditional mosaic, these mats can easily conform to a sloped or drain-pitched concrete bed or built-up drain pan. While the use of mastic is a fine alternative for backsplashes and low moisture environments, the shower floor requires a really durable product like cement based thin set.
Another reason I like to dry fit ahead of time is thin set only has a fixed “pot life” and once it starts to become too stiff the installation becomes more problematic. Rushing a pebble tile installation will only lead to poor results. For pebble tiles to look and feel good (the rounded stone tops act like little foot massagers) the install depth needs to be just right. The size of the stones will vary but most like to bed about 1/8” deep. If a small amount of thin set oozes up through the mesh backer you will get a great long lasting and secure bond. If a lot oozes up then the thin set is competing with the “grout zone”.