If you have an HVAC register above your tank, how will it effect the tank is what I believe you're asking; correct?
I don't think the potential for dust floating in the air will be the much greater than anywhere else in the room. And it is easily removed through filtration if it does find its way into the tank. You could also place a filter over the register as an added layer of protection.
Where you might see a noticeable difference is in temperature/evaporation. During the winter, heat coming from a register directly above the tank would accelerate evaporation causing the tank to cool. You might find yourself going through water from your Auto Top Off (commonly noted as "ATO") at a greater rate than in the summer. Also, you may notice the heater in your tank coming on more frequently from the cooling.
A plastic deflector can help with much of this.
Hope this info helps.
Dom