MrReds:
Your query is vague and not specific enough. You don't state your concern (if you do have one) and what you identify as a problem area or a hazard.
There is nothing specific about the word optimal. The "best" pressure drop is the one that works for your application. If you want to optimize, you should study the effects of reducing the pressure drop(s) - such as more capital costs due to larger equipment, more maintenance, etc.
There are definite hazards involved in applying excessive pressure drops across evaporators when using hydrocarbon refrigerants - such as propane, ethylene, etc. This should be an obvious deduction to users. A below-atmospheric suction pressure at the compressor's suction flange will induce the migration of air (& oxygen) into the refrigerant's circulating system - principally through gaskets, etc. This is a hazzard because the subsequent compression of refrigerant+oxygen raises the mixture's temperature to a level where you could get a combustion reaction. You don't want to go there. That's why the vaporizer's pressure (& temperature) are kept at a level that ensures that the compressor's suction pressure will always be gauge-positive. The need for purging the accumulation of non-condensables should serve as a hard reminder of the fact that all refrigeration systems suck in air - through time and sooner or later. It just happens and it should be addressed.
There isn't the same concern for the condenser since it normally always operates at relatively elevated pressure and the leakage is normally always outwards.
I don't know what you mean by "a preferable circuit disposition".
I hope this helps address your concern.
