No bandwidth is saved by putting task servers at each site, so you may want to place them at HQ. Have you referred to the planning and implementation guide for sizing information? How many nodes do you have, at how many sites?
I know that if the NS holds your Software Library, it won't also replicate it to a second location. I have to assume a site server would behave in the same way, but I haven't tested this.
Are you planning on placing your two package servers at your large remote sites?
If you plan on imaging at a site, you'll want to have site servers running task, package, and DS handlers there.