According to BASF the only times you cannot treat is if precipitation is currently happening or if the soil is over-saturated with water, frozen, or in conditions where runoff or soil movement from the area or treatment site will occur. It would be great to not have rain after your termite treatment, but certainly not something that is “needed (or a must)” for the product to be efficacious or provide long-term residual activity/structural protection.
To apply Termidor SC to a home with a pier and beam foundation, you will need to trench around each pier. You will need to trench around any point where a support for the home touches the home. If the piers are hollow blocks you also have to drill a hole in each pier to treat the inside space where termites could potentially travel up without hitting the treated soil around the pier.
You can apply Termidor SC as long as the ground is not frozen and make sure that it does not rain for 24 hours minimum after you do your treatment.