Spinach is somewhat akin to rhubarb... i.e., if it's flowering it's too late. Best time is just as the leaves are full and thick. Flowering is a prelude to producing seed and the spinach becomes tough and bitter, in my opinion...
Don't Let Them Bolt
When the heat of summer hits, the natural tendency of cool-weather crops such as flowering brassicas and leafy greens is to reproduce and go to seed, or bolt. Pick these vegetables before they bolt and become bitter or otherwise unusable.