If you wanted to be all-encompassing, you'd need to factor in at least the following in no particular order.
•Aesthetics
•Allowable Stress
•Pocket acceptability
•Vibrations Risk
•Interference
So, a water pipe in the middle of nowhere with a low flow rate and no risk of hammer could have an acceptable sag of "whatever doesn't fail."
Conversely, a high velocity line in a crowded rack with high visibility could have an incredibly small allowable.
As such, my opinion is that designing for a specified sag value across the board can be unnecessarily costly, or it could result in under-engineered piping as described in the above bullet points.
With that said, most of the time, this is indirectly specified through maximum piping span charts, which are based upon deflection and/or sustained stress values.