Is it acceptable to use 1uF decoupler on a 7805?
In theory, we want the output of a 7805 to be 5V, no matter what. The regulator surely does its best, but there are two things that it needs to react to:


output current demand
input voltage


Ignoring the output for now, when you switch on the input, the voltage rises with a certain speed, and if that speed is faster than the internals of the 7805 are designed to react to, it will lag in regulating the voltage down, leading to higher output voltage than desired.

Usually it should not happen to really go that far beyond the 5V, but it is not totally inconceivable.

You can hide that voltage spike by adding more output capacitance, but how much is mostly a thing of experimentation. You should also add some good input capacitance to slow down the rise of the input voltage to give the regulator more time to react.

If you worry so much about the output voltage not going out of spec, there are some things you may want to consider:


protect it with a zener
use some kind of switch to only switch on when the voltage is stable
use a better regulator, possibly one with a shutdown/enable pin that you set up with an RC to go on when the input voltage is a bit more stable.