Dull (non-shiny) solder joints are usually the effect of coarse grain structure in the solid solder joint (though there can be other causes). The more slowly a joint cools, the coarser the grain growth and, conversely, the faster the joint cools, the finer the grain growth and the shinier the joint. Dull or grainy joints are not a big concern – the grain structure will grow in a joint over time anyway. But having a dull joint is giving this eventuality a “head start”. However, under no circumstances, should a joint be touched up by applying a soldering iron (or other means) simply for the cosmetics of a shinier joint. The damage you may be causing by inducing accelerated intermetallic growth will far offset any gains in joint strength through a finer grain structure.