All a matter of how much you want to try and push your N900.
AFAIK, "noone" had their N900 die flat due to overclocking as of yet.
Also, before it dies flat, it will crash, hang or reboot anyways. =P

Lowering voltages is to reduce the stress/heat on the CPU.
So just try everything at stock default voltages first.
Then try lowering the voltages at the same frequencies if stable using default.