it depends on the employer...and the job. i was "smart" enough to choose a position where i absolutely cannot miss work during certain weeks of the month. at my last job, i showed up with f'n pneumonia for a couple days because of the workload involved with that job. i eventually got the hell out of there but some employers are like that...ridiculous workloads make it almost impossible to get caught up after you take time off.
my new job is much better with that...but there are still certain weeks of the month i just can't take off so i have to plan all my vacations around it.