As the layout moves over time with contraction and expansion (yes, even in a climate controlled environment) anything like wood filler or spackle is just going to crack becoming visible again, even through your ground cover. Best to use something flexible when dried like a caulk, liquid nails etc etc and then cover the joint with masking or duct tape, then paint over it all with a thick layer of latex paint. Even if that joint does crack or separate over time, the tape will keep it concealed below the ground cover.
We used tape on some joints and not on others. The joints without tape eventually showed up on the surface through the ground cover. This is more likely to happen in states like CA where the ground is prone to shaking from the slightest of earth quakes. Most of you don't have to deal with that, but expansion and contraction yield the same result over time.
