It depends on the amp... some will handle 18V with out a sweat, but above 16V is probably overdoing things for most amps unless they specifically state otherwise... of course your gain adjustment and listening habbits also play into this...
Most amps will also cut out below 9-10V because the op-amps do not behave properly below that input, and the amp will go into protect... but the notion that you can blow an amp because of voltage drop is rediculous...