Helpful to make sure everyone knows their responsibilities ahead of time- I would say it's good to talk it through by first preview. Checking to make sure that walkie-talkies are charged, routes are clear, etc is a great way to fill down time during tech.
As Maggie mentioned, it varies- some theatres already have a plan in place and you just need to review it with the team. House management is a part of any performance evacuation so it's not always the SM making the announcement. (In some theatres, the sound system cuts out if the fire alarm is going off, so you might not be able to make a VOG announcement.)
If there are weapons in use at the time, especially firearms, it's important to have a plan for how those are secured when evacuating. It's also helpful to have a list of all actors and crew handy, so that you're not trying to remember how many folks you're looking for.
We had a fire alarm go off toward the end of Act 1 of a show I did a few years ago, and one of the most memorable things about it was one of the ASMs walking onstage, taking an actress by the shoulders, and walking her off to evacuate- she hadn't registered what was happening.