Before garnishing food with flowers, make sure that the flowers are not a poisonous variety and that they are pesticide free. Your best option is to use garden grown flowers or those commercially raised especially for eating.
Just because a flower is considered edible, or nontoxic, doesn't mean it tastes good. If you are using edible flowers as an ingredient rather than a garnish, sample the flowers first to make sure that they can compliment your dish.
Below are some of the common edible flowers and what they taste and smell like.
Squash blossoms - sweet nectar Roses - floral Violets - very floral Carnations - bland and somewhat bitter Bachelor's Buttons - bland Lilacs - light floral Lavender - lemon perfume Nasturtiums - pepper, radish, watercress