Many shoe stretcher manufacturers recommend doing so (and, not coincidentally, happen to sell these sprays or even bundle them with their stretchers). Typically, stretch spray is applied to the inside of the shoe and allowed to sit for a few minutes before starting the stretching process. Just be sure to read the instructions before using the spray to make sure that the shoe material won’t be damaged by the pretreatment. Be especially cautious with softer leather, suede shoes, and thin materials, as a spray might soak through and stain the outside.