I do not see where it is recommended to carry it in SA. You can do as you please, but if a firearm has a specific safety feature, it is meant to be used. Some have more that others. Some designs are better than others.
3.3 Decocker (P99 AS)
The decocker offers a safe way of de-cocking the P99 AS. The decocker is routinely used to de-cock the striker. De-cock the P99 AS after loading, before holstering and prior to disassembly. Press the decocker downwards into the slide until the striker is audibly and visually de-cocked (3.3 Fig. 1).
If you find that it is difficult or cumbersome to manipulate the decocker using your shooting hand’s thumb, or if you are left-handed, you should use the thumb of your non-shooting hand to do so.
Always point the pistol in a safe direction during de-cocking.
Always de-cock the P99 AS before holstering. The de-cocked pistol remains ready for service using the Double Action trigger
WARNING: Pulling the slide rearward will cock the striker again.