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The extruder only has to be as hot as is needed to extrude filament and not clog.
This will be different for different filaments, and for same filament but different brand, and for different colours. The difference is caused by the different additives added. You ned to experiment with each new reel and fine tune by a few degrees. Personally never been below 185 for PLA - jams.

Clear PLA is ideal at about 190C (Aside - use a thermocouple not a thermister. Thermister is not an absolute reference temp, not as accurate). The ideal printing temp is the lowest one that will guarantee a non clog because its too cold.
Two main reasons support this:

  1. Any material if "very" hot will shrink as it cools and this will deform your part, or delaminate, or pull away from bed. Do not underestimate the effect. Extrude at lowest temp possible.

  2. The newly extruded material needs to bond to the existing material. Materials like PLA re-melt easily and you get a good bond. Some materials need more heat to re-melt the previous layer and bond. Alas solution is to slow print head(heat leaks from head to pre-melt existing), not to increase temp. This means slower print but adhesion is good as new layer is well bonded to previous layer. Some people don't like that and want it to be different - too bad. Physics! Going really hot and really fast will result in deformed, delaminated, less precisely dimensioned, object

  3. Leaking - maybe important depending on extruder design but if its too hot it will leak as flow is high. A retract can be done before every move but slows down print and is not ideal anyway. Extruder has to be well designed and tested by lots of use. Its actually very difficult balance to get this right for different materials, you need thermal breaks in the feed path and all kinds of design elements need to be solved. Look at failure of last Makerbot smart extruder thing for example of company that should have known better.

Can't think of a material that needs 300C. Remember, higher temp than material needs, means shrinkage as it cools. No escaping this.