Typically not, however there are a few things to consider. Usually we do a greater pitch to the drain. Rule of thumb is 1/4" per foot, with pebbles we increase that slightly to create better flow of the water to the drain. When the pebbles are sponged after grouting it is desirable to see some of the definition of the stone, we avoid burying them in grout. This creates little troughs around each rock, therefore water can get hooked on the edges and not flow as nicely. Doing the increased pitch gives you the best of both worlds the beauty of the pebbles as well as adequate pitch for water to flow properly to the drain.