I wouldn't worry about CS5 harming any workflow as CS5 mostly just adds some features on it doesn't change anything major from how it always worked in CS4. I currently have both CS4 and CS5 on the same computer with no problems so you should be fine too.
The two are seperate programs and will not interfere with each other.
However, if you do not want CS4 remove it BEFORE you install CS5. If you remove CS4 after installing CS5 some users have problems with file associations were program want to open everything in CS4, and it is not present.
Oh yes good point, i had this problem before too, but managed to re-assign the open command to use CS5 eventually, but its easy to get rid of CS4 first if you know you won't be using it anymore. Again, i really wouldn't worry about CS5 being different from CS4 its really not for almost all tasks.