Having a wired machine can make configuration setup or changes a bit easier to manage. For example, changing a security feature on the router while connected wirelessly could boot you off and unless you know what to do on the client explicitly then you might be getting out that ethernet cable anyway but you are correct. There is no requirement for wired nodes when using a Router/AP combo.
As far as OS's. If you plan on using computer names when browsing to other nodes on the lan or (shudder) using network neighborhood/places, then you might consider turning off the master browser function on the 98 machine. Here is another suggestion. Don't run any network wizards, although (someone can correct me if I am wrong) XP home may require at least the initial wizard to enable some networking features. Don't really know enough about XP home networking I guess but I don't use it and don't really support it.