It depends on the preparation.
I have the Creative Whip which comes with a little recipe book and not all of the recipes demand that you chill after charging. For example the "instant chocolate mousse" is really instant. The only liquid ingredient is the cream itself which is already chilled, and the gas expansion inside the canister chills it even further, so it's ready right away.
On the other hand, if you're making something with gelatin, carrageenan, or any other gelling agent which needs to set, then you'll obviously need to fill the canister while it's still a liquid (otherwise it's going to be rather hard to whip, not to mention hard to get into the canister itself!) and then chill to set it.
Basically it's simply a question of how cold your liquid is before it goes into the canister vs. how cold you want it to be when you serve it. If you can chill it to the correct temperature beforehand, and that won't prevent you from whipping it (as it would with an actual gel), then by all means chill first.
In the specific case when you're using creams and/or cheeses to make a mousse or espuma, you almost never need to chill it in the whipper itself - although it won't hurt either.
By the way, if you're making a gel or anything else that needs to set, it's best to chill it gradually in an ice water bath and shake it frequently as it chills, in order to keep the gas dispersed. If you just charge it and stick it in the fridge, you may find it very hard to dispense afterward.
Heavy Whipping Cream (at least in the States I believe) has a fat content of 36% or greater. Light Cream generally sits between 30% and 36%. Heavier cream generally will have a "richer" taste. Light Cream will also not whip quite as thick and not hold it's form as well. That said, for most cases, I've successfully managed to use them interchangeably. I guess it might make a difference if you were doing something where holding a rigid structure for extended periods of time was important.
Note: What various grades of cream/dairy are called varies by region... Here in Canada we have Whipping Cream 35% and Heavy Cream ~40%. If I were somewhere where "Light Whipping Cream" sat closer to 30%, I might grab something a little higher.
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Best Answer
Sometimes whipping cream separates a little bit in the package and forms a solid layer on top of the liquid cream. This is normal and should not effect its whipping properties.
The few times I have had a hard time whipping cream has been on really hot days or when the cream doesnt contain enough fat. The cream has then separated in the same way as you described. Your cream should have about 40% fat.
You should easily be able to tell if the cream has gone off by smelling or tasting it.