When having a cake tasting, do you offer more than one frosting base?

For example, you can use ABC, IMBC, SMBC, or Bettercream (Rich's or Frostin Pride) and flavor them to your needs.

Is there one that's preferred or expected?

I always provide my vanilla and chocolate cakes since they are the base of all of my recipes, my normal buttercream,  a sampling of my marshmallow fondamt + allow potential customers to select 2 additional frosting flavors and 2 fillings from my menu.

I do a vanilla cake with ABC, chocolate cake with my fudge filling, almond with amaretto IMBC, and a  fruit or other flavor with a meringue buttercream to coordinate. That way people can see if they prefer the ABC to a meringue or vice-versa.

Personally, I only do one style of icing-IMBC. I don't like making and working with ABC, and there are literally millions of other places folks can get it.  IMBC pleases 95% of the people 95% of the time, and I'm fine with losing out on someone who prefers something else.  That's just me, tho!  I make 3 favorite flavor combinations of cake with filling.  They're the most popular for a reason.  Then people can expand and customize from there if they like. I will also try to accommodate a flavor request.