most physics texts, if not all will use SI units Jantimot
the mixing of units (say kilos and grams) in a physics text will be specifically done to illustrate the importance of unit conversion, nothing else
yes it is important to know though as in the real world - which is engineering you can cause a lot of problems if you mix up for example your litres and gallons
just like that jet airliner that was not filled up with enough fuel and ended up crashing
