It is best to make sure the zipper is all the way open when ever you are sitting on it. The problem is not so much the coils or teeth being deformed as it is the coils or teeth pulling off the ribbon. Once that happens the zipper is kaput. Zipper tape is surprisingly strong but the bonding process is not always perfect.
By only opening the zipper part of the way you create a different stress on the tape and _that_ is what will lead to zipper failure. Open it all the way and you should not have a problem.