Should I seal my hot leftovers before putting them in the fridge?

You should NOT put hot food in the fridge. It warm the whole fridge (affecting other foods) and increase need for energy to cool everything down.

Puting a lid on container help it seal itself (hence the ventilation button in more costly containers to ease the opening) due to air contracting when cooling.

Put a lid on container when putting hot food in. Wait for the container to get warm and then put in the fridge.

Edit from non-US point of view with sourcef from manufacturers.

Never Put Hot Food In the Fridge, Here's Why
Liebherr advices for storing hot food
Goverment of South Australia advice (look at temp 21 °C and advice " when food stops giving off steam it can be placed in cold storage")

Part of the challenge here might be your definition of "hot." When I think of "left overs", I see what is left over after eating a meal. It is certainly not "piping hot", as in just off the stove. There is no problem packaging, covering, and refrigerating these items. As an alternate example, when I make a batch of chicken stock, once it is cool enough in the pot to deal with so I don't burn myself, I portion it into deli containers for freezing. I cover (but sometimes don't completely seal, depending on how hot), and leave on the counter for 30 minutes to an hour before moving to the freezer. These are just examples.

It seems to me that (as at least one link in another answer here states) waiting until your food is room temperature might be problematic, as this could easily place it in the danger zone for some time.

Putting a lid on a deli container is only a problem if the items inside are so hot that the lid bulges. The issues is not food safety, but that the lid might be compromised or unseal.

Sealing does not cause spoilage. Spoilage bacteria do that under proper conditions of temperature and time.

Bottom line: Place your leftovers in a container, seal them, and refrigerate as soon as you are able to handle them, and within food safety parameters.