Well obviously it can't be bit perfect to the music if its playing two sounds at the same time. Does that really matter though? You wouldn't be listening to two separate tracks at the same time.
If you just don't like the system sounds interrupting your music then WASAPI in exclusive mode is a good way to automatically disable them when listening to music and automatically enable them when you're done. With normal direct sound or wave out the extra sounds stop affecting fidelity as soon as they stop playing so it doesn't really hurt anything assuming you actually want to or don't mind hearing them.
At least in 7. From what I understand about XP the mixer works like that most of the time but is more prone to doing weird things like using SRC for no good reason and using its terrible algorithm which is just designed to be super fast only good enough to play the system sounds on top of whatever sample rate you're using. At least that's what I've heard. I never had a problem with it though and I only use XP with my work computer right now.
Ah ok yeah that's what I've been wondering.Looks like I'll abandon the WASAPI method then...it's a chore having to close Foobar whenever I wanna watch youtube or anything, if DirectSound is playing bitperfect whenever I dont have other things playing then yeah that's the most optimal thing for me.