Now. They will speed up cycling, for one thing. You can use a number of things to dip plants. Bleach is very harsh, it can melt some, especially if left for more than a couple of minutes. A common dilution is 1 part bleach to 19-20 parts water, dip for no more than 2 minutes, rinse thoroughly, use extra dechlor in tank.
Hydrogen Peroxide can be misted on at 3%, the usual drugstore strength, and need not be rinsed. It degrades to oxygen and hydrogen, so it can even be spot squirted on a problem right in your tank.
Potassium Permanganate, if you can get it, is safer than bleach for the plants. Opinons differ on how safe it is for humans, but it is a powerful oxidizer just like bleach and oxygen are, so a dust mask is a good idea. It will stain skin permanently, so cheap gloves are a good idea too. A pinch of it to a bucket, you only want the water to turn a medium pink colour. Dip for a few minutes, rinse well.
Safer by far is alum. You can get it in the spice aisle, or some bulk food stores. It's used for pickling, among other things. One or two TBSP to a gallon, you can dip overnight. Rinse well. The weaker solution is likely safest for very delicate plants, either way, it's much safer than the oxidizers are.
