We usually go for the 3 order rule, but in the case of areas and volumes, for dimensional reasons, dam and hm make it into the three order rule. Dm (or dam) is not common but dam^3 has some uses, the same goes for hm, hm is used for only special situations (like meassuring train distances), but hm^2 is almost globally used for big chunks of land. Also, with hm^2, we always keep the unit, so for example, Parque Nacional Iguazú in Argentina has 67620 hm^2 (also ha or hectarea).

I’m also an engineer and I generally despise imperial units, but I have to say that inchs are pretty handy and the 1 in = 25,4000… mm relation is pretty neat