Tom Sloper, an apparently very experienced American player, writes about etiquette. It turns out that "One person's bad etiquette is another person's official rule" - so you have to adapt. There is a huge difference between tournament play and casual games with friends. Also, you need to consider house rules. In the end, there is a definite answer to your question in the rule book, but its usage in practice depends on the circumstances.
DISCARDING AHEAD -- When I play with my Japanese friends, they yell at
me if I claim a discard and then make the exposure before discarding
(nobody can play until I discard, and they want to PLAY!). But when I
play with my American friends, they call me dead if I discard before
making the exposure. One person's bad etiquette is another person's
official rule! Why it's a bad habit -- Do it with players who strictly
follow the rules (like in a tournament) and you'll be called dead.
PICKING AHEAD -- Many American players think "picking ahead" (AKA
"playing with a future") is the correct way to play. But when these
players come to a tournament... watch out! Old habits die hard. And
picking ahead is the surest way I know to get called dead in a
tournament. In case anybody's been living in a cave, picking ahead is
a no-no. That other mah-jongg association (I'm not naming names, but
their initials are NMJL) HATES picking ahead! Read their rulebook and
yearly bulletin sometime if you don't believe me. Why it's a bad habit
-- (1) It causes too many conflicts when someone wants to claim a discard. You might think that it makes the game go faster, but it
really doesn't. (2) Do it in a tournament and you'll be called dead.
He adds a few more points to consider (including "avoid making everyone wait"), it's a very interesting read.