If it gives you peace of mind, you can. When you are checking out, remove your hypoallergenic pillowcases and place them in a Ziploc bag so they don’t touch the other items in your suitcase. Wash them thoroughly as soon as you get home. Another thing to consider is size. Since you probably won’t know the size of the hotel pillows, opt for king-size pillowcases. They might be too big if the pillows are smaller, but they won’t be too small to use.