As long as you take the other weeks of pills everyday on time, then you are safe for the week you take your placebos. You should not have to use a backup method while you are taking the placebos either.
As for not getting pregnant after your period, that is false. Women ovulate at different times of the month, and ovulation with sperm is all it takes to get pregnant. It is even possible to get pregnant while you have your period-it is not likely, but it could still happen.
Always take your pills at the same time everyday and never skip one single day. As long you follow the directions to the letter, the pill is 99% effective.