Wall-mounting and stands gives freedom to mount at the height you want. The RT251's have a good bass response provided by their rear-firing port built into the speaker's back plate, which doubles as a convenient keyhole wall mounting bracket. So, Polk implies wall mounting is okay. Both wall mounting and stands allow the speaker is be aimed where you want it and towed where you want it as well. Some pro users like stands because you can move the cabinets away from the walls more and let the speaker breath better minimizing early reflections.
Depends on the intended use. Will you be using them as mains or surrounds? Stands would be preferable for mains, they will allow you to toe-in the speakers and place them in various locations to achieve the best perfromance and imaging in your room. Wall mounting main speakers will not allow that kind of flexibility.