Hi scubasteve0011,
It is not safe to go into deco without training, but it does happen. On my admittedly short number of dives I have not gone into deco, but with task-loading it has happened to friends of mine. The short answer is that, with a computer, you are in pretty good hands because it will give you the correct deco regime for the physiological models with which it was programmed. The longer answer is that a given physiological model may not apply to you, or may apply to you most of that time, but not on that one critical dive. Nitrogen bubble formation is still poorly understood, despite decades of study. The principal reason for this is that human bodies are highly variable form one model (of body) to the next, and from one day to the next. So, try to stay out of deco (unless trained for it). If you go into deco, and your training does not indicate otherwise, trust your computer and (after surfacing) reflect on how to stay out of deco next time.
The most obvious reason for staying out of accidental deco is, by definition, you have not planned for a deco dive. This likely means that you do not have a surface O2 delivery system and have not planned an evac to the nearest hyperbaric chamber. If you do get DCI (and this can happen even with a no deco dive), the most critical factors are 100% O2 at the surface and a speedy transportation to a hyperbaric facility. Hydration also helps.