To act out of turn once is ok, we all do it (or have done it) but to continually act out of turn - then that is just plain stupid (annoyingly so)
It is up to the dealer to ensure that all cards are visable and to control the action - if it is dealer dealt then i dont think that there are two many excuses for acting out of turn!
I the Poker rule book that i have (its a bit old - but generally still applies) it states that:
"the following actions are improper and gounds for a warning, suspension or baring of the violator:
1) Delibratly acting out of turn
It is annoying - but if someone wants to muck their Button - SB - BB before the action gets to them....... thats there loss!!!