It's critical for at least one airline, too.
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Your flight is booked, bags are packed, and you're ready to roll on your next great vacation. Everything's set for your journey to the airport — but hang on. Did you check into your flight? If you haven't already, stop reading this story, go check in immediately and come back when you're done. We'll wait.

You're back? Phew. That was close. Because, according to Scott Keyes, founder of Scott's Cheap Flights and author of Take More Vacations, you should always check into your flight the second the option becomes available.

"There are a few instances where it is quite important to check in early — and the earlier, the better," Keyes shares with Travel + Leisure.

According to Keyes, if you're flying with certain airlines, this can matter just a bit more. For a prime example, he points to Southwest, as the airline assigns seats based on a first-come, first-serve basis.

"That first-come, first-serve is determined by what time you checked in for your flight," he says. "So, if you check-in for your flight, even five or 10 minutes after that 24-hour mark, you're going to likely get put into the B or C-section groups, which means your odds of sitting together as a group or getting an aisle or window seat are not great. Especially if it's a packed flight."

According to Keyes, whenever he takes a Southwest flight, he makes sure to set an alarm or calendar reminder so he can have his app open and ready for check-in.

Family doing self service check-in for flight at the airport
Credit: Jacob Lund/Getty Images

Another reason Keyes warns that it's important to check into a flight as soon as you can is due to the risk of being late to the airport and not being able to check-in on arrival. For example, American Airlines won't let you check into a flight if you arrive within 45 minutes of takeoff for a domestic trip and within 90 minutes of takeoff for an international flight, meaning you either have to ensure you can really get to the airport early or just put your mind at ease by checking in beforehand. As for checking in at the airport at the last second, Keyes says, "there's essentially no good reason to be putting it off until then."

Beyond ensuring you can get on the plane, Keyes also notes there are other goodies to be had by checking in as early as you can on an airline's website or app.

"Sometimes airlines allow seat selection after check-in or at least have first pick compared to folks who check-in later," he says. "It's rare these days, but every once in a while, check-in time can be the tiebreaker for determining upgrades too."

Keyes says, sometimes, it can even be the tiebreaker for who gets bumped from an overpacked flight. He notes, this too is extremely rare, but hey, why take your chances? But, since you've already checked in like a responsible traveler, you've got nothing to worry about, so go ahead and make your way to the airport bar and toast to your responsible traveler habits.