Sometimes, your shoe size includes a half size, but the boutique shoe you want only comes in whole sizes. Depending on the fit of the shoe, you can size in either direction. If the brand runs large, order a half size down. If the brand runs small, order a half size up.
In cases where the brand runs true to size, consider the materials and usage of the shoe itself. Some materials, like leather, will stretch a little over time, so you can size down. However, shoes designed for long-term wear mean your feet may swell over the course of the day, so you should likely size up to accommodate any swelling.