All Mac Pro can run 10.6 and 10.7.
Don't forget, the current Mac Pros are identical to the models available 2 years ago before 10.7 was ever released. The most recent "update" only changed what CPUs are offered, everything else remained unchanged from the 2010 models.
NO Mac Pro requires 10.8 or 10.7 to operate at full capacity.
10.6.8 is the best OS choice for all Mac Pros unless you absolutely need 10.7/8 software compatibility. Its the most efficient version and it doesn't have any iOS garbage.
To add to that:
Dual boot (or triple) if you must use Lion/ML and keep 10.6.8 on your drive.
Lion+ can using VMs run SL as a guest in order to avoid dual boot and have access to apps using Rosetta supported OS.
10.7.5 though does provide support, and 10.8.2 takes it further, for Nvidia GPUs like GTX 570 w/ 2.5GB or the newer 670 and 680s (keep in mind the Mac only has two 6-pin power cable sockets to the PSU).
The "2012" w/ 3.2GHz 4-core is a good solid base system able to support up to 48GB RAM. No SATA3. Still uses PCIe 2.x. Can accept W3690 (DIY).
Does not support network boot via EFI though so buying or including DVDs and 10.6.4+ OEM or retail (the ATI 5770 needs 10.6.5+).
For every "rule" there is the exception that makes the rule.