Can I use a tactile switch as computer power switch?
Assuming that your PSU is an ATX one, 


  PS-ON is an active low signal that turns on all of the main power rails including 3.3V, 5V, -5V, 12V, and -12V
  power rails. When this signal is held high by the PC board or left open circuited, outputs of the power rails should
  not deliver current and should be held at a zero potential with respect to ground. Power should be delivered to the
  rails only if the PS-ON signal is held at ground potential. This signal should be held at +5VDC by a pull-up resistor
  internal to the power supply.


Source: ATX Specification - version 2.01

So if your PSU is indeed an ATX supply, PS-ON seems to be a high impedance input. In that case, and if the power switch is connected directly to your PSU, then there shouldn't be a problem pulling it to GND using a tactile switch.

However, the power switch might be going through some logic or buffering on your motherboard, so you might want to look up the specifications for your motherboard if you want to make sure it's safe. I suspect it will be safe, but without knowing your motherboard there's no sure way to tell.

See also:


Using PWR_OK to Activate an ATX Power Supply Automatically
Does an ATX PSU&#39;s PS_ON Signal Require Resistance