I have my ammo stored in my closet in ammo cans. I'm going to put a small dehumidifier in the closet. Should i leave the ammo cans open with the dehumidifier running for awhile before i seal them up?

I have no idea where you live, where your closet is located and how humid your closet gets.

That said, what I would do is open the cans on a very dry day. Run the Dehumidifier too, and at the end of a few days of dry weather, seal the cans. If they are US Military Ammo Cans they should have a water and moisture proof seal. My experience with them is that even after many years, they still seal up tight. I suppose it would no hurt to run the Humidifier and leave it on a reasonably dry setting as well - but if the cans seal well, I personally would leave them closed.

If your cans do not seal well and you have other ammo just in boxes and the air is very moist then I'd leave the ammo on shelves (not in closed cans) and set up a Humidifier to run on a regular basis.

I wonder how that can of FA50 M2 Ball I just cracked open for my club's Garand Match was packed and stored for the past 68 years. Every round went bang when it was supposed to.

Leave them sealed or leave the boxes on the shelf. It won't make any difference. Spend the money on more ammo, not a dehumidifier.

I can’t imagine why you’d spend money on a dehumidifier for a closet in which you store ammo, whether the ammo is loose, in cardboard boxes or in any other container unless you’re worried your clothing in the closet will get moldy. You sure don’t need a dehumidifier for ammo.

Modern ammo (ammo loaded after the First World War) is pretty much waterproof. I wouldn’t store ammo in standing water but if your closet has standing water in it, you need a drain not a dehumidifier.

Spend your time and money improving your shooting skills, not obsessing about dehumidifiers.

I think you are overthinking this matter. I have stored factory and reloaded ammo in a closet for nearly two decades and it is still 100% reliable when I take it to the range.

Ammo isn't that susceptible to humidity...even handloads.
Heat is more of an enemy. As long as the ammo is in your house and the temperature stays below 100 degrees most of the time you will not need a dehumidifier .
I have recently shot 30-06 , 45 acp and 9 mm Luger ammo , from WWII dated 1944 (that's 74 years !), my dad had simply sat the little cardboard boxes on a closet shelf in his house, and it all fired just fine.
All this took place in Louisiana....Humidity , you bet your backside we got humidity.
Remember ...it's not the humidity , it's the heat you want to avoid .
Gary

I've run reloads through my washing machine on accident after leaving them in my pockets. They've all gone bang. Unless you've got condensation on the walls of the closet and plan on storing the ammo in that environment for years I wouldn't worry about it.