This is a difference between theory and practice.
I remember when I was beginning my studies I had lots of problems to understand why every teacher takes $\pi$ as 3.14 and not 3.1415926..., as I learned in school. In algebra $\pi$ was never calculated and the results were something like $2\sqrt2 \pi$. That was because in engineer calculations we don't care about so much precision.
In simple theoretical circuits we make assumptions as well. Because voltage drop coming from Ohm's law is very small, we consider it zero. We are not interested if the current in main circuit is
1 A or
1.00089 A, 1% accuracy is enough for almost all engineering purposes.
For practical calculations we are even not able to consider all impacting factors, while some (with different impact) are:
- influence of temperature on resistance,
- the fact, that resistance is not distributed regularly along the line length, so $dR / dx \neq const$,
- influence of humidity, insulation resistance which leads to current flows from the wire to earth,
- magnetic fluxes from other sources,
- internal and external capacitances,
- magnetic field of human brain of the scientist (why not include this?),
- effects of gravity of Jupiter (theoretical physics says there is such impact),
Usually calculations are made to know what current (eg. short-circuit current) will flow. If we do calculations to find what switchgear or fuse we need, it doesn't matter if the current should be
15.23213121 A or
15.23943 A, because we choose
16 A or
The real element (a wire) could be considered as being something like this:
This is an equivalent scheme of wire. The resistance is not the only parameter here, so is inductance of the wire (because it forms some kind of loop and is producing magnetic flux) and there is internal capacitance between both ends. You may say that for DC circuits
C do not matter, but it is not true. In transients states they have quite large impact on the current / voltage, especially for high frequencies. In electronics even short wires can create many bad phenomena (that's why it is now not possible to make a microprocessor with frequencies larger than some GHz), that cannot be easily handled.
But in "normal" frequencies like 50/60 Hz or DC (which we assume to be constant) this really doesn't matter.
R) can be safely omitted and we still get almost perfect results, and exact for the purposes we are doing calculations for.
However, if the line is quite long, it is not possible to omit these values. By "long" I mean hundreds of kilometres/miles and there is a special theory of dealing with long lines. The higher frequency we use, the shorter "long line" is and for electronics a long line can be even some millimetres.
So, finally, answering to your question: we are justified to say that a wire is $0 \Omega$ resistance (or rather: impedance) if and only if the error made by this assumption is not disturbing required accuracy of calculations we do.
We are never justified if the impact of this assumption is violating our desired accuracy.
In theoretical calculations (eg. in school) you are to learn that $U = I R$, so you should pay attention to this and nothing else; this is a hypothetical example, however as all the physics is. The real world "violates" theoretical physics: there are no elastic collisions, there is friction, and there are no ideal conductors.