Most times the rear spokes will come a bit loose as the spokes stretch. With the rear wheel being dished and one side of the spokes having more tension to pull the rim towards the drive side. Plus all you weight on the rear and power going through the pedals to the rim does make the rear wheel more prone to needing a bit of tweaking here and there.
I have left it alone in the past and trashed the odd rim with being lazy and not being arsed to do it! I knew the teniosn had gone a bit in the rear and then done a bumpy, rocky, and fast peaks ride and trashed a couple of rims.
Best and simplest thing to do is stick the wheel in a truing stand or make one out of your frame some blue tack and an old spoke with a nipple on the end which will give you a good idea of how true the wheel is while you make adjustment.
Just work around the wheel giving each spoke half a turn with the spoke key. Give the spokes a feel and repeat if they still feel a bit too springy. You might have to give the drive side spokes a quarter turn more than the none drives side though to keep the wheel dished ok.
Don't worry too much about the roundness of the wheel, small adjustment evenly made as above won't knock that out.
