How many incompletes in a post are still ok?

Hi, I'm using Grabit to download files. I have the program set to show me incomplete files in Red text and was wondering what is the maximum number of incomplete parts you can download while not sacrificing the file. I understand how to repair with PAR2s, just don't want to push my luck. I often see a file made up of lets say 100 complete parts and then there are 3 or 4 incomplete. Is this still worth downloading?
Thanks for your time.....

Most sets that have pars usually have 5 to 10 percent in pars...

As long as there are enough PAR's to repair, every single file could be incomplete.

here is a good explantion of par files...
www.slyck.com/ng.php?page=6