Some motherboards in my experience do not handle upping voltages very well. It can cause serious problems if a motherboard doesn't implement it very well or isn't really designed to run memory at a certain voltage.
Again, it's probably not going to hurt anything. But it can, even if by user error mucking around in the BIOS, and I don't see any real gain by overriding the motherboard. Memory only makes a noticeable difference with varying capacity, but memory speed and latency rarely are worth bothering with, especially on the Core 2 line, which aren't very memory sensitive at all. ANd if he isn't upping the voltage to better any of these, it matters even less.
I'd only recommend upping the voltage if the memory is unstable at a lower voltage, but not just because it's rated at 2.1V.