Firstly let's correct a slight misunderstanding: we don't have a single "latest" SW version for any of our switches.
RA.16.02.0014 is "latest and greatest" only in RA.16.02 -branch. We have multiple currently active branches (RA.16.01, RA.15.18, RA.15.16m) and all of them have their individual latest version. Same applies to all our ProVision/ArubaOS -based products. Which to use depends entirely on your situation and needs - choose the branch accordingly. It's impossible for me to decide for you, but here's something to consider.
You're currently running RA.15.05 in some switches. Let's assume you're upgrading because you want to introduce some new capabilities which are only available on RA.16. In that case - by all means go for .16.01 or .16.02. But running this old FW versions imply that you're more interested in stability, just want to get the devices on the same FW level for management reasons, and don't want to upgrade too frequently. If this is the case, the Maintenance branch RA.15.16m might be more appropriate solution.
The functionalities of a Maintenance branch are locked, we only fix issues. The next time we want to add new features to a Maintenance branch, we will designate a new one. For example we might in the future decide that we want to change RA.16.02 as a new Maintenance release. It will have all the features it currently has (and what are added until that point) but new ones will be added to other branches. This means that a Maintenance branch has a lot longer lifetime than a CPE release, and upgrades are published far less frequently, only when necessary.
Note that this is not a statement that 16.02 will be the next Maintenance branch. I just chose it as an example how this works.
What comes to upgrade paths; they are always listed in the Release Notes. Currently Release Notes are available from RA.15.06 upwards. So just check out the latest RN on these. If intermediate steps are required, they will be listed. If there's no list, there's no limitations either. Likewise the RN contains a full list of available enhancements.
Personally I would do a long jump such as from 15.05 to 16.xx with a couple of intermediate steps regardless of what the RN says; and always upgrade the devices always via the secondary flash. But that's me... I rather err on the side of caution.