Windows Live Movie Maker, the plug-in and YouTube will take care of everything. You'll only need to opt for YouTube when it's time to publish the movie. That's the good part.
What's the rest of the story?
YouTube has two quality levels, 'normal quality' and 'higher quality'. If the quality of the uploaded movie is high enough, there will be a quality option under the YouTube player. The fully automated Windows Live Movie Maker process doesn't produce movies of sufficient quality to make it to YouTube's higher quality level and there is notably less visual quality.
Your videos on YouTube will have another quality issue... I'll illustrate it with the last image below. See the black borders on all 4 sides of the uploaded movie from Windows Live Movie Maker. The source file was a widescreen DV-AVI file made by Movie Maker 2.1, the highest quality I can make, yet the video only plays on YouTube in 'normal quality' with no option to notch it up.
The quality issue isn't caused by YouTube... it's simply processing the quality of the uploaded file, which is set by the defaults of Windows Movie Maker Live. It's still in its beta stage, so things might get better by the time it's formally released.
There is one thing about YouTube that lingers on. Uploaded wmv files with stereo audio come back as flash files with mono. It's been that way in the past. This version of Movie Maker creates files in stereo, and YouTube continues to convert them to mono.
If you want the higher quality and no black borders, go to my website at www.papajohn.org and see the Distribute Movies > YouTube page. Get and use the custom profiles for Movie Maker 2,1 in XP, Movie Maker 6 in Vista, and Photo Story 3. Windows Live Movie Maker doesn't have a feature to use custom profiles, so you'll need to continue using the other software.
If stereo is important to you, use other services such as vimeo, which doesn't have a plug-in for Windows Live Movie Maker.