Many people assume that a custom start page should be the home page in your browser to make the best use of it, but that is entirely a matter of personal preference. The idea behind a personalized page is to bring many different tasks you might perform on the web all to one central place for you.
But some start pages can take a while to load. If you find that your custom start page takes a few extra second to load, and you also find that you are often loading up your browser just to do a quick search for something, then having Google or Yahoo as your home page is perfectly fine.
After all, you can always bookmark your personalized start page and head there first thing in the morning.
I don't use my custom start page as the default page in my browser. I actually use two different personalized pages, and because I use Firefox as my main browser, I have buttons on my toolbar that will take me to either of my start pages.
But I don't like using them as my default page because I often boot up the browser just to do a Google search on something.