​During a dive with my dive computer, my buddy and I will turn the dive based on the first limit we reach: remaining no-stop time or air supply turn point. Depending on the dive conditions and depth you sometimes reach one limit before the other.

Why should I not turn off a dive computer between dives?

Don't turn your computer off between dives.

Most won't let you, but if you take out the battery or shut the computer down, it loses its memory of your previous dives and your residual nitrogen. You'll have to allow all residual nitrogen to leave your body before resuming use of the computer.

When using a dive computer I should ascend at a rate?

For example, old PADI dive tables (based on the US Navy Dive Tables) allow a maximum ascent rate of 60 feet/18 meters per minute. In these situations, it's usually safest err on the side of conservatism, so our recommendation is to never exceed an ascent rate of 30 feet/9 meters per a minute.