I have folder for each version
If you create a project it will create an _EmtpyPorject folder with a blank project you can use to see what dll are supposed to be in the project
If you keep it seperate you can not only compare the 8.0 bin dll to what you have but also what should be on youir 7.0 project.
Sometimes you upadate and forget to delete stuff.
Also I keep my languages files in the corresponding version.
So no - My personal answer would be to keep it in seperate folders!
See step 1. b a on that page. This is easy for me if I keep it in seperate folders.
Just my 2 cents