I should use the 64bit testing distro for packaging?
Ideally, packages should be built and tested on both
32 bit and 64 bit. The SRPM itself is "architecture neutral" so it doesn't matter which environment it is built on. Hopefully, the same SRPM will build and work in both environments but sometimes some adjustment is necessary to achieve that. Personally, I do the initial packaging on my normal 32 bit system, then once it is tested and working I transfer the SRPM to my 64 bit system and check that it builds and works there too. You can, of course, use VirtualBox rather than having separate machines but it can be a little slow for large packages. If you really can only have 1 environment it is best to make it 32 bit (and then ask others to test your package on 64 bit).