Yes, car washing, pollen, dust, heat, rain... They all degrade a wax' life (and the degree of degradation varies from wax to wax). If you're washing it with a PH neutral car shampoo, then it wont degrade all together in one go but there will be some degree of degradation with each wash. In case of NXT, while it provides good looks on (mainly a lighter colored car's wax) , but in my experience it has been the quickest to degrade in Meguiars range with its looks dropping on a daily basis and protection going in 2 weeks.
Also, be careful for a velvet cover... While it feels soft and plushy, it will trap dirt inside due to nature of its nap (which upon dragging the cover will interact with cars surface, causing swirls in the long run). You'll be needing to washing the cover more often.
For regular (daily) dust accumulation (non sticky to the surface), you should use a Quick detailer to remove that safely. The QD has lubricants which (along with a microfiber) minimize the drag effect of dust on cars surface and thereby not causing swirls easily. It also restores a just waxed look. All the more important to have for a black car.