We will be travelling to Spain in October and early November. Is there a necessity for reservations or advance ticketing at that time?
No, We recently traveled over New Years and had no problems getting ticket the day before.
A good reason to book in advance for train tickets, depending on which routes you're following, is to get major discounts of up to 60% off. Some of the AVE routes can be quite expensive, eg. Madrid-Barcelona and Madrid-Sevilla. You can book up to 62 1/2 days out at www.renfe.es for print-at-home tickets.