Is it worthwhile to reuse old rims and/or spokes?

Hi,

I've recently completed restoring my beloved Schwinn Aluminum road bike from 1991. I found a fine wheelset online, which I'm currently using, but it has Shimano 105SC (1055) hubs while the rest of the bike is using Shimano 600 Ultegra (6400) parts. I already have a fitting 6400 hubset, and I...