In a climate like Southern California, which gets little rain, drainage isn’t a concern for me. (Though we did receive several inches in our recent set of storms, and the tub held up well with no waterlogging.)
If you live in a very rainy climate, it might be a wise idea to drill a few more holes in the bottom to help with root aeration.
I don’t recommend adding stones, styrofoam, pot shards, or any of the other fillers people often turn to for pot drainage.
This is a long-standing gardening myth that refuses to die, despite simple physics proving that so-called drainage materials do the complete opposite of what they’re intended.
Water does not move easily from finer textured material (soil) to coarser textured material (say, stones). As the water trickles through the soil and reaches the stones, it will stop and start to pool in between the layers (in effect, “back filling” the pot) until the soil is fully saturated.
Only then will it start to drain — a slow process that keeps the roots sitting in soggy soil far longer than it ordinarily would if the whole pot were filled with soil. Rather than assisting with drainage, the stones restrict it.