I remember when we did arrangements of Berg's four pieces for clarinet and piano in instrumentation class. This was towards the end of 2005, but the performance was 2006, so we cheated a bit and wrote 2006 on the scores, since Berg died 1935 and the copyright was in tact till 70 years after his death
Of course, this was an official project by a conservatory with several participants, so you need to be a bit more careful then when a single person (unless well-known) does a solitary arrangement of a piece.
But you can't
always count on the fact that copyright owners don't mind anyways. That may be true for many composers themselves, but publishers tend to be quite possessive of their music.