<p>When using a heated bed with your printer, I have seen claims of running temperatures of 90c throughout the print.</p>

<p>That seems like a fairly high power use to keep a large slab of, say, aluminium at 90c for long print times (ie multiple hours).</p>

<p>Is there a common 'sweet spot' for operating temperature?</p>

<p>Does it depend on material?</p>

<p>Is a heated bed required?</p>

Googling "what temperature for different filaments" gives a few good links, but the top link looks golden.
https://filaments.ca/pages/temperature-guide
They have temperature guides for both extrusion temperatures and heated bed, as well as suggestions for better adhesion. I'm not that experienced, but their information is similar to information I've seen elsewhere.
Their suggestion for PLA is 215-235 degrees Celsius and a bed temperature of 60 to 80 degrees. That sounds a bit hot to me but every brand (and type) of filament will perform best at different temperatures. I've had problem getting nice bridges at 210 degrees, but had excellent results at 190.
For ABS they say 230 to 240 degrees with the bed at 80 to 100.
Use these values as a starting point, when they fail you make an educated guess about what went wrong and adjust (one parameter at a time) til it works for you. Find a calibration object you like and trim the temperatures so it prints the best your printer can. At this point in the technology experimentation is a large part of making things work.
One thing that's important to remember is that the temperature that you set your bed to and the temperature that your bed gets to is not the same thing. Depending on the construction, assembly and quality of your heater the actual temperature can differ anything from almost nothing to twenty degrees or more.