The answer is : YES PostgreSQL round values.

Thanks to @vérace I can found it into the source code of PostgreSQL where the d2d function is used to output the decimal form of the value.

This is quite complicated but to simplify, a validity interval is calculated depending on the exponent and the mantissa. Then the decimal value is rounding in consistency to the validity interval.
The final result is rounded to about (really about) the value multiply by 10^-16.

This answer will be marked as "accepted" when I will be granted to ;-)