Are audio expanders usually used in mastering?

They can be... Not typically I suppose. I use frequency-conscious expanders frequently for a lot of classical work (especially live recordings).

It really depends on the situation. I suppose I'd hope to not need one on any given project...

Interesting,,,
John,are you familiar,or use,the Distressor from Empirical?

Used them - Don't have them.

Not too much to say about them... I'd like them more for mixing than anything else.