One issue with using all GPIO pins is that some pins must be held at certain state during power-on, while others can have a moment of glitch during power-on. For example, GPIO2 must remain high at power-on, GPIO15 must be low, and GPIO16 starts as high but flickers for a couple hundred milliseconds. So these GPIOs, when connected to relays, can produce moments of false action of the relays, albeit this only happens at power-on or reboot. If this can be tolerated, then you can use all GPIO pins.