I know a lot of people say you should wait but is there any big thing that makes it a bad idea to just go in immediately?
Yes. The CO doesn't just go from zero to super high instantaneously. While you're sitting in the tent with the CO rising, you will be subject to all of the "negative and undesirable" effects from the CO as the level increases through all of the "lower levels", like headache, nausea, vomiting, convulsions, burning eyes. This is, basically, how it used to be done when people used to use their car engines for the CO source, but it was even worse because of all the other noxious substances someone had to breathe in while waiting on death. In order to attain a "more peaceful" exit using CO, best practices dictate waiting for a maximum level to be achieved since, hopefully, it will be at a high enough level to render someone unconscious within a few breaths after entering the tent. This way anything that follows leading to death will not have to be consciously experienced. Also, by only having to take a few breaths to become unconscious (levels at 10000 ppm or more), there will be less chance of "backing out". Backing out of CO short of death, after breathing it at lower levels for a while, can leave someone with serious, debilitating, life-long injuries, including severe neurological impairment - think nursing home being spoon-fed and having your ass wiped by someone else.
Make sense?