Checking can be a problem for tires exposed to UV rays for long periods. It can shorten their life, so I do use covers.
Being cheap though, and hating the look of those off-white commercial tire covers, I make mine out of BBQ covers. One cover split down the middle (lengthwise) takes care of both sets of tandem wheels. A few grommets and an hour of time and voila! Bungies in back hold it nice and tight, and the colour range gives you a much nicer appearance. Mine are slate grey and matches the trailer skirt quite well.