You will know what look you are most comfortable with.
Probably best to avoid a haircut the day before the shoot though, a bit of time between cut and shoot may offer a more relaxed look and avoid the risk of cancellation due to a haircut ‘issue’.
People with longer hair may wear it both up and down offering sometimes startlingly differing looks in a single session. Feel comfortable to experiment during the shoot.
Very short-cropped hair may benefit from a trim on the morning but be sure to clear away any trimmings before the shoot and, if you routinely shave (chin or head), do so before the shoot but try to get a good, tidy and complete shave.