Yes there is a chance that they will come up and yes it is worth teaching your child. Every mark is important with regard to codes (and maths) as this will give your child some leeway with regard to unknown vocabulary.
They can appear with the following types...
If the code for DUST is WFHF
What is the code for POLISH
AM is to ZN
as BR is to ?
You could also see a reversed/inside outside mirror as in
AM is to NZ
as VD is to ? (WE)
For long and 2 letter codes [C and U] my children do not start the section until they have set the alphabet up for mirrors. They draw a line between M and N [the mirror] then put 5 on top of E and V [to help with the counting of letters away from the mirror.]
In doing so, it reminds them that there could be some mirror/inverted mirrors included.
They then check the whole section for mirrors before they start, putting a large M next to the relevant mirror question. It only takes a few seconds but saves time and heartache in the long run.