Cloudee - it needs to clearly state it is optional within the AWB for it not to be mandatory.
You will see that is a standard text included in AWB's - many AWB's refere you to other data often the manufacturer and those publications may be "Optional", "Recommended" or "Mandatory".
CAsA did have a policy that recommended means mandatory (piston engine TBO's confirm this as the manufacturers only give recommended figures)
So if the manufacturer states the data is optional - then you don't need to do it. If they say nothing, recommended or mandatory - then the AWB needs to be done.
* So the AWB dose not make you do it - The Regs do, on grounds that an AWB is approved data issued by CAsA.