Is there any positive N on which it's agreed among astronomers that 10^-N is accepted as 0 (zero)?
No, there isn't.
Astronomy overlaps with cosmology and there can be some pretty big and small numbers.
However most physical parameters also have units and the units chosen affect the numerical values.
A great example is the Jansky:
The jansky (symbol Jy, plural janskys) is a non-SI unit of spectral flux density, or spectral irradiance, used especially in radio astronomy. It is equivalent to 10^−26 watts per square metre per hertz.
In some cases one might even imagine going the other way and using L☉/ℓP² myr (solar luminosities per square planck-length per million years) which would require numerical values about 1x10^-110 times smaller.
Instead, astronomers and other scientists usually decide is something is small enough that it doesn't matter within the context of the current discussion. THey use words like negligible and small to indicate they're going to ignore it.