Is it true that petroleum jelly is good to minimize scars from insect bites, rash and open wounds?

Yes indeed!: The only thing really shown to help the healing process and minimize scarring is keeping a wound moist and covered. Most scar products accomplish this, however so does petroleum jelly. In two randomized comparison trials of mederma and petroleum jelly, mederma did not improve the appearance of scars any more than petroleum jelly, and vitamin e doesn’t do any better.

It's OK: Wounds heal best when kept moist and vaseline is a good moisturizer. However, there may be products more appropriate for specific wounds. For example, wounds with a heavy bacterial load may do better with an antibiotic cream such as silvadene (silver sulfadiazine) while insect bites might need an antipuritic/antiinflammatant agent. Your doctor should be able to help you make individualized choices.