Pure heroin is actually a very safe drug (from an OD standpoint). If you can safely calculate your dose everytime, you will not OD. However, street heroin is by no means pure, and most OD's you read about are people who get cleaner, less cut batch than what they are used to and take what is their "normal" dose, not realizing they are shooting hot and taking far too much than their bodies can handle.

edited to add: The lifestyle of a heroin (or any drug) addict is not something anyone should ever experience. From the few heroin addicts I know, their lives are basically black holes of hopelessness. Living for a drug is not living at all.