of course you should ask for and confirm any time off before you take it!
it seems by the tone of the message (yet again ....) that maybe people have been taking the ****. really your employer shouldn't have to even tell you this once, let alone more than once as it surely is just common sense? Their business necesstates it's employees being there - thats what having a job is all about. What would happen if every single member of staff decided they were having an appointment on the same day and didnt confirm the time off prior to being off? Even when you are sick, common sense (and job cntracts) dictates that you phone n BEFORE the start time of your shift.
They are under no oligation to grant you time off for dentist appts, medical appts etc