What if I didn't charge my iPhone 5 before using when I first got it?

So I just got my first iPhone ever and didn't know that you're supposed to charge it for 8h first before beginning to use it. I charged it (even though it looks fully charged) as soon as I got home but as it was charging I sent out a few smses. Anyway so I never took it off the charger but Mum says that cos I already started to use it, it will never work at its optimum anymore.
Is that true or is it still safe since I used it while it was still charging?

It is totally safe. They even use it when you go to the store so they can set it up for you. Don't worry about using it while it's charging, i do it all the time.

You should be fine, I would always charge it up overnight but just do it now. I always use my iphone while charging also and have never had a problem with the battery.