The main stresses will always be concentrated in the spar and the spar center joint. As such I see the center sheeting adding more to the torsional rigidity in the center section to help support the leading and trailing edges. I don't really think a couple of short peices of sheeting is actually adding much to the spar's load bearing capability. Oh sure, it adds something around the center joint but it's very minimal.

So assuming a good spar design I really don't think there's a lot to be gained by adding a full fiberglass wrap. All it needs is a well crafted butt glue joint. For larger models there may be some advantage to adding a light band of glass tape but I don't see any reason for multiple layers. Once it's as strong as the balsa anything extra is just added weight.

There is one exception to this. In some designs I've seen the wings are built as two separate panels using a full D box with thick center ribs. These panels are joined like a big handlaunch glider with no carry through joiners. This sort of method uses the skin strength of the D box to carry the loads. In this case a good center joint wrap of light wieght cloth using a narrow and wide layer to avoid the stress risers as Olllie points out is the way to go. The sheeting IS the spar in this case and the wrap IS the joiner.