Since minions are a metagaming concept, then I see no problem informing players what are minions and what aren't.
As a player, I am weary of my DM pulling the boss-disguised-as-a-minion , or worse, minion-disguised-as-a-boss. Because this means the DM would be banking on me wasting my Encounter and Daily powers on a minion creature. I should be the one to make that decision based on pure information. If I wasted my good powers based on intentional faulty information, I'd be pissed at the DM making such a metagaming ploy.
Because of this concern, my gaming behavior has been altered to a very conservative style of play. I only use my At-Wills until a hit has confirmed that a creature is a non-minion. Only then will I use my Encounter or Daily powers.
For some, this might seem the opposite of the free-wheeling, seat-of-your-pants kind of play 4e appears to promote. I've applied this style to advising my teammates. I point strikers and those with area effects at specific targets to "test" their minion-ness. For me, each fight has become a kind of "hunt the non-minion" metagame anyway.
If DMs hide their minion-ness, then expect this kind of behavior. If DMs openly expose their minion-ness, then expect a more brave and free kind of tactical play.