Originally Posted by Thrik
Beating with rank 100 basically just gives you a short bonus movie after the game has ended, so you can safely complete the game to 100% and then finish it as normal. You'll still get to see the normal ending and
the bonus movie.
It's a short video though, and definitely just a little bonus for those determined enough to get rank 100. If it's going to take you ages and ages to do it I wouldn't bother. It's not really meant to be something you know about until you get it.
When you hit ranks after 95 you get the message "RANK UP! Bonus Scene at Rank 100!" So there's plenty of notification from that point on.
If you really want to see the video it's on Youtube. Search for Psychonauts and bonus.