Personally I have been using an external audio interface on my main PC for years as the only audio device, FOR EVERYTHING, without difficulty. It is much easier to send everything out to one set of monitors/speakers that way.
I have generally kept my onboard audio disabled in the BIOS. I recall trying to use both audio drivers at one time, but that became impractical for me.
One weird thing that I have noticed, is that when using one app that grabs the ASIO driver, such as Sonar, is that if I start another app such as Reaper that needs ASIO, I get the "no driver available" alert.
But Windows, browsers, and other media apps seem to get along fine as long as they are not configured specifically to use ASIO.