Better not to set the df_max_version unless you *know* that it's incompatible past a certain point.
I thought df_max_version was a safety feature to prevent users from messing up their save files. If someone tries to drag a graphics pack from an older version of the pack into a pack for a newer version of Dwarf Fortress, it will prevent it from working unless they edit the manifest file (at which point they should realize they are taking a risk).
Newer versions of the pack will include updated graphics packs that have been checked for compatibility with the latest Dwarf Fortress and will have their manifests updated to reflect that.
The idea is to allow old packs to be used by PyLNP for old installs, not require manual intervention with every update
Are you talking about dragging the "Dwarf Fortress" folder out of an old version of a Lazy Newb Pack and into a current version? I guess you would need to drag the contents of the old baselines folder into the baselines folder of the new Lazy Newb Pack too. It seems like setting a max version would be helpful for people doing something like this, so they don't mistakenly try to use an older graphics pack with the newest Dwarf Fortress.
The max version also works well to mark if a pack has been checked yet for compatibility with the latest version. And if a pack hasn't been updated in a while, it allows you to see the latest version of Dwarf Fortress it was known to be working with.
I'm not seeing the disadvantage to setting this field to the latest known version of Dwarf Fortress that a graphics pack works with. It seems to be helpful to keep it put the latest known working version number there.