[DevonDiver]

As far as I was aware, the only really need to roll tanks and mix gases comes when using Helium. If you are only dealing with nitrox mixtures, then there should be no stratification of the gasses (and if there was, they'd probably mix themselves during the course of transporting the cylinders to your next dive site).

As for long term storage of the tanks, then there are several potential issues to consider:

1. Tank pressure. Have the tanks leaked at all...is the pressure suitable for the dives you are planning to conduct?

2. Gas mixture. There will be no issues of the gas 'going off' and neither N2 or O2 will leak from a tank at a greater rate than the other (with Trimix, the Helium can leak from a tank over time, changing the mix). You still need to re-confirm the O2% before diving the tanks though.

3. Tank deterioration. Storing high O2 mixtures for long periods will increase the opportunity for oxidisation (rusting) of the tanks to occur IF there was any moisture inside the tank. There could be internal corrosion. However, if your tanks are still in O2 Service AND you are happy that your tanks were initially filled with clean, dry, air and that no moisture could have entered the tanks since the last testing (you never completely drained them)...then you shouldn't have a problem.

The only reason for draining the gas would be in order to conduct a service on the tanks, if that was required or if you had any doubts about moisture/corrosion occurring. As for the stage regulators...just consider their condition, last O2 service and how they've been stored. If anything needs sorting out... it'll be the regs.