Do I need to heat treat a wedding cake knife?

[elementfe]

Marriage is for life, and the same is for the cake knife, I would certainly HT it properly.

That's right! Taking a shortcut on the knife is like getting married in a khaki shirt and corduroys. Do it right!
You could use carbon steel- if you throw in a nice little case and a little vial of oil it'll keep as well as non-treated AEB-L and you can do the treat yourself. Be sure the couple know the whole story, it'll be something to tell years later.