It depends on order of mismatch. If you put 1000 Ohm 0.1λ it must be considered, because effect is very significant. For such huge mismatch effect vanishes around 0.001λ.
The 1/10 lambda rules means, you can analyze transmission lines as lumped circuit (series L, shunt C) below this length without causing large errors. But the lumped elements must be still considered, their effect can be very small or large, depending on the impedance relations.
If you are in an impedance matched design, it may be easier to use transmission line calculation methods even for short lines.
A load Zl is seen different when it is looked through a tl. There is a famous formula saying zin= zo*(zload+jz0*tan(teta))...
As you can observe, if teta is small enough(the line short enough), zin becomes zload. When the line length increases, the load start to be seen different.