The great thing about mattress and box spring encasements is that there is no single best time to use one.  They can and should be used all the time. 

Bed Bugs in a bed that is already infested will be trapped in and will die.  No additional bugs will be able to get in, and a bug cannot feed through an encasement.  They can be used before an infestation is found, upon finding it, or after treatment. 

A mattress and a box spring provide endless places to hide and in very close proximity to a next meal.  Some benefits to using bedding encasements include that pesticides will not be needed on the bed itself in the event they are used at all.  Encasements eliminate hiding places for bed bugs, and make them easier to detect.  An added bonus is that for individuals with allergies or asthma, these bedding encasements limit exposure to dust mites as well.

The best mattress and box spring encasements will be made of a material that will not rip or tear, and is comfortable.  Cheaper encasements may not have these qualities.  Most experts recommend Protect-A-Bed.

Every bed should be encased, but those with a high turn-over rate such as those in hotels, shelters, multifamily housing, dorms, and low-income housing absolutely require bedding encasements.