An article in the Telegraph argued that there is no point in placing pebbles or shards in the bottom of whatever, the reason behind it I cannot repeat (something to do with capillarity). But as it rains like hell where I live, and the containers with shards keep having better crops than those without I won´t say anything else.
But definitely, add some sand. If the place your flowerbeds are gets a lot of exposure to rain, you´ll regret you haven´t. Too sunny in summer? All right, be content with one fifth grits, mixed well along very good compost. Because even the best brand will get soaked with frequent rain, and your plants will suffer.
After planting, don´t forget to mulch. If you can´t find dry leaves in bulk, buy something like decorative stones or bark chips. This will help your plants keep moist and in the right temperature.