I prefer to dead-head, just for the sake of neatness. Some dianthus (especially those called 'pinks') will re-bloom if you keep them watered and deadheaded. Be sure it's planted in sandy soil, with really good drainage; and give it a light-weight mulch over-winter... you may have one of the perennials.

I'd advise no mulch - if it's perennial, especially one of the ones with bluish leaves, a mulch over the crown may well kill it. At best, you'll end up with long stringy stems with tufts on the end and no foliage closer to the crown.

I grew various species dianthus and their cultivars for years, and never mulched them. When they were accidentally "mulched" by leaves from other plants, they always looked awful afterwards, and I usually had to take cuttings and reestablish the plants to have a decent looking mat of foliage.

I don't actually know of a dianthus which is NOT winterhardy in Virginia zone 7, but there are a number which don't like our summers. Something that came in a pot as a gift, and survived the winter, sounds like one of the miniature carnations. They're a bit touchy here, but given good drainage and air circulation, are sufficiently hardy to make it.