[CentOS] Building a 64bit rpm

I want to build the 64bit version of Xen 3.2, they only show one srpm available. I assume if its compiled on a x86_64 version of CentOS 5.1 it will be a 64bit rpm when its done?

I am aware its not good to build as root but out of curiosity, if I used a vm that I would later destroy, is the only caveat to building as root potential damage to the build system, or can the actual rpm be built incorrectly?

Thanks for any hints,
jlc