I don't glass the center section of ARF wings. The most common argument for doing so is wing failures. Most ARF wings are pretty strong. A common cause of ARF wing failures is a lack of glue on the plywood joiner(s) and root ribs. Doing practice assemblies before actually gluing (so you know it will go together right), using a slow setting epoxy (30 minutes or more so it has a chance to soak into the wood and to give enough working time), and slathering plenty of epoxy on the root ribs, on the joiner, and in the joiner holes, will ensure that the center section is not going to fail.