There is no reason not to re-use a sound spoke. I have done so repeatedly with great success but you do have to pay attention to where they go. If you look at the bend in the spoke at the head, you will see that some are bent more than 90 degrees and some are bent less. This indicates in which position on the flange the spoke was previously installed, inside or outside. If you are careful to put the spoke back in the same position then there will be no problem. Less bend goes inside the flange, more bend goes outside. It is not that hard to just sort them into two piles before you start to build.

It is also a good idea on the rear wheel to position the pulling spokes on the inside of the flange. They will be under the most tension when riding and less bend is better for that spoke position. You see wheels all the time that are not done that way but it is not proper in my opinion.