Validate is widely regarded as the worst daemon to trigger. Adding another guard to the level is usually more harmful than the effect of any other daemon; but it's worse than that, because the guard is an alerted Enforcer who throws scan grenades to detect your agents behind cover. The effect is so severe that the mere possibility of Validate should make you think twice before hacking a device with an unknown daemon.
It is therefore usually best to avoid triggering Validate, especially early in a mission. You can use Hunter or Taurus to remove Validate from a device you need to hack, or if appropriate use an EMP Pack or Dr. Xu's Subdermal Tools to temporarily disable the device instead of hacking it.
If you must hack a device with Validate — or if you cannot resist the payoff of doing so — then you should normally wait until you have completed any other mission objectives, found the exit room, and made a plan to get your agents there. If the guard entrances are near where your agents need to be, it may be a good idea to put agents next to them, ready to knock out and pin the Enforcer. Alternatively, if you have an agent in place before triggering Validate, you can distract him by sprint toggling.