PLA starts to change its properties at above its glass transition temperature of 60-65 °C, if stored there too long. Keeping it at 160°C, close to the melting temperature (173-178 °C) can degrade the material relatively rapidly. During an extrusion, this is usually mitigated by filling fresh material into the melt while the older material gets extruded, but keeping the machine stagnant at the high temperature has not only creep the heat up from the designated melt zone (thus preheating material that should not be preheated yet) but also can damage the material deep in the melt zone. Together with this possible material degradation, the preheated material melts faster and might overshoot the aimed at print temperature until the normal temperature is reached again. Both effects can lead to reduced print quality in the lower layers.
The time saving from keeping the filament heated up for an extended period of time is, in my opinion, not worth the quality reduction that can come from using non-uniform material. You pay more in lost prints than you save in time for heating up the head.
If it is impossible to not keep the hotend heated between prints, it might be advisable try these:
- Starting every print with a larger purging operation might get rid of degraded material. It would be as simple as extruding some quantity of material before starting the actual print at the edge of the build plate.
- Aside from purging, it might be a good idea to reduce the hold temperature from 160 °C to a lower temperature, allowing the melt to partly solidify and keep the heat creep in check.