Hi bbunney13,
Assuming that you decide to pull the trigger … congratulations on your new mattress
As you know I think you are making a great quality choice and I’m looking forward to your comments and feedback once you’ve reveived it and have had the chance to sleep on it for a bit.
There is more information about dust mites and dust mite allergies in post #2 here and in post #3 here and there is also more information about mattress allergy encasements (that surround the mattress) vs mattress protectors (that are like a fitted sheet and are easier to remove) in post #2 here. There is also more about the pros and cons of different mattress protectors and some examples of each of them in post #89 here.
Mattress allergy encasements are meant to be used on a semi permanent basis and aren’t meant to be easily removed so I would agree that it would make sense to at least hold of with using an encasement (if you feel you need one) until you are confident that you won’t need to rearrange or exchange any layers in your mattress.
Phoenix