I have been using zen-cart for 1 year and more.
And operating 2 sites with it.

My paypal IPN was setted to 'off'.
And I am using a same paypal email for those 2 sites.

I never got any errors. And have processed nearly 10000 orders.

I have no idea if it's neccessary to set the IPN at paypal's profile page.

And am I missing any function of zen-cart?

I am using 1.30 zen now.