In the words of the great romans.......Duh. If you had a doubt would you want to be reassured, or would you want her to blow you off with a tough girl act? Put yourself in her shoes, while the insecurity is mostly her problem, a little reassurance is like relationship insurance.
If her insecurities get intrusive, then it's time to put the breaks on and politely tell her "I have chosen you, I'm with you and your who I want to be with....... so what is your issue". I actually had that exact phrase used on me by my husband after we met, as I was getting jealous over a friend who later proved she wanted more. Anywho, it provoked thought and I agreed, if he loves me and wants me, what is my major malfunction.
Give a little love here, and pet and reassure her.
Best of luck!