I am having a long time discussion with a buddy about the no deco time shown in a computer. How close can you get to the zero time while diving, without risking too much. Some opinions are to go up when you reach the 10 min mark, some consider the 5 minute mark.
To zero.
But the closer you get to zero:
* The longer you should ascend. There is no definitive guideline, but an easy rule would be to make your safety stop mandatory if you get to 5 min NDL, and add a stop minute for every next minute. So at zero your 3-minute safety stop becomes a mandatory 8-minute one.
* The more air you need yo have when you begin your ascent. This is both to accommodate the longer stop and to provide compensatory safety margin. Probably begin mandatory ascent at 1/3 tank pressure as you remove the NDL margin.
NB: "Ascent" in this context means dive over, going to the safety stop, not just to a shallower level of a multi-level dive site.
* The more important air supply redundancy is. If you're getting very close to NDL, you need to have a RAS (just a small pony) or at least be very close to a reliable long-term buddy for the last minutes of the dive.
* The better you should be at handling emergency deco. At a very minimum, absolutely learn what ceiling means, where it is shown on your computer, and practice simulated deco if possible.
The reason for these precautions is that things happen, and if you routinely get down to 0 NDL, you will sometimes go below, into deco. You should absolutely know what to do if you do. There's a lot of stories about recreational divers that send their computer into deco mode and then screw themselves up, not knowing what their computer is telling them.
Another reason not to go too close to NDL is subclinical DCS - minor damage that manifests as post-dive fatigue. You'll feel much more tired after repeatedly diving down to zero NDL than after multiple conservative dives 10 minutes short of your NDL.
This can be mitigated by treating near-NDL ascent as deco at low GF, and some computers can be set that way. I'm assuming strictly recreational (no-deco) diving for everything above. Planned deco itself can be quite safe and easier on your health than riding NDL up to zero - if you have all four of gear redundancy, pre-dive gas planning, technical deco training, and a well-communicating team (and very unsafe if just one of these elements is missing).