If the site is to simply keep competitors from obtaining them, I would just register the domain and leave it parked. I would not run domains just for misspelled combinations. You start to have too many domains that you have to get to rank which could dilute your efforts for the real site.
How much traffic do you get for the misspelled version, and where does your main site rank for those misspellings? I might try PPC for those misspelled versions and just park those domains. To get them to rank you will have to build content and links. Lots of work for what may be little return. With PPC you can target just the misspelling.
If anything, I would 301 the misspelling to the main site and hope that it will bring the maine site up in the SERPs for the misspelling.
I hope that helps