Most people consider 5-10% safe. Going from 1.37v, that means 1.44 to 1.5v.

However, of equal improtance is the cooling. I admit I am not as familiar with Xeon specs, but I can imagine that since it is a server-class CPU it's meant to handle a bit more than a consumer grade chip. Just make sure that whatever voltage you decide to put into it, that it stays well under the rated temps spec'ed by Intel for your CPU.