Quite simple, my Dear: That depends...as usual!
Single-Breasted suit: Can be left unbuttoned, if there is no representative occasion like greeting someone or opening up a meeting, then done up of course, BUT: Always leave the lower button undone, full buttoning up of a single-breaster indicates ignorance
Double-breasted suit: Always buttoned up, leaving it undone looks just extremely uncool and dumb!
No jacket at all: In a meeting, but always let the guy at the top decide, or the client, if they say "let's get rid of the jackets", it's ok, but you can keep it on if you like, that wouldn't be considered impolite, the other way round (taking it off while everybody keeps it on) is very impolite.
Does that help?