This applies to annuals, perennials or vegetables. For example, if the only difference in appearance of two plants side by side is one with tight buds and one in flower, go with the plant in bud. Or, if the only healthy plants are in flower, be ruthless and cut off the blooms.
Plants suffer less transplant shock and develop roots faster if they are without blooms.
New buds will appear in two weeks or so. In addition to buds, look for plants bearing vigorous, well-shaped leaves with a bright rich color. Leaves should be free of insects and disease symptoms. Also check for signs of pests on the stems, particularly near the soil line. If you see something that looks abnormal to you, ask a staff person.