Using Megawizard functions vs. instantiating in SOPC Builder

If you want a master in your SOPC Builder system to access something through Avalon and if that something is in the SOPC Builder component library, then try first to use the built-in SOPC Builder component. If you create your own design block or something like a MegaWizard variation (whether a simple megafunction like a FIFO or something like a memory interface MegaCore without using what the MegaCore's user guide calls the "SOPC Builder flow"), you will need a custom component to interface it to masters in your SOPC Builder system. I've known of someone using a custom component to connect Nios to a memory interface MegaCore, but I'd avoid that unless some special capability is necessary that you don't get using the SOPC Builder flow.