When to start maternity leave??

How early should I start my maternity leave? I'm about to start my third trimester so I'll be talking to my boss about it soon. She's very flexible about working with me and giving me all of the time off I need. I plan to take 12 weeks after the baby is born. This is my first baby, due on March 23, normal and healthy pregnancy so far. Planning on returning to work around the middle of June.

My question is, do I need to plan on taking time off a couple of days before baby's due date just in case? I know all birth experiences are different, baby will come when he decides to and that can't necessarily be planned around, but I've also read that most first time moms deliver after the original due date.

I work retail at a small boutique and there's normally just one employee working at a time so it's not like if I were to go into labor at work there would already be someone else here, especially during the week. If that were to happen I would have to close the store and I don't want to leave my co-workers scrambling if baby decides to come early. Should I be safe and start my maternity leave at the beginning of the week of the 23rd? Or would I be okay to work until his due date and then start my leave?